Roasted flat mushrooms are a simple side dish where large portobello caps bake under a blanket of soft butter flecked with green onion, parsley, and fresh thyme. Foil keeps the herbs bright while the mushrooms turn tender and juicy, and the buttery juices pool in the tray for spooning over the top.
With only 5 minutes of prep and 25 minutes in the oven, this dish fits into a busy dinner plan in about 30 minutes, yet it looks right at home on a holiday table. It pairs with steak, roast chicken, or pasta, and it also works as a vegetarian main served over toast.
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Why This Roasted Flat Mushrooms Recipe Works
Large flat mushrooms are naturally meaty, and their wide gills hold the herb butter in place like little bowls. As the butter melts, it seasons the mushrooms and mixes with their juices to form a savory sauce.
Covering the tray with foil traps steam, which keeps the herbs green and makes the mushrooms tender all the way through. There is no need to flip them or stir the pan, so the oven does the work.
The garnish of fresh green onion and thyme sprigs added at the end gives a fresh finish and makes the dish look as good as it tastes.
Ingredients for Roasted Flat Mushrooms
- 8 large flat portobello mushrooms (about 4 to 4 1/2 inches wide), stem tips trimmed
- 7 tbsp unsalted butter, softened
- 1 cup thinly sliced green onion
- 1/3 cup finely chopped fresh parsley
- 1 tbsp fresh thyme leaves (or 1 tsp dried thyme)
- 1/2 tsp kosher salt
- 1/4 tsp black pepper
- 3 tbsp chopped green onion, for garnish
- 12 fresh thyme sprigs, for garnish
Flat portobello mushrooms. Pick caps of similar size, about 4 to 4 1/2 inches wide, with firm tops and intact gills. Trim only the stem tips so the caps stay whole.
Unsalted butter. Soft butter spreads easily and melts into the gills. Using unsalted butter lets you control the seasoning.
Green onion and parsley. These fresh herbs give the butter color and a mild oniony bite. Slice the onion thinly and chop the parsley finely so the butter spreads smoothly.
Fresh thyme. Thyme adds an earthy aroma that pairs naturally with mushrooms. If you only have dried thyme, use about 1 teaspoon.
Salt and pepper. A modest amount of salt and black pepper seasons the whole tray, since the butter and mushrooms carry plenty of flavor.
How to Make Roasted Flat Mushrooms
1. Clean the mushrooms. Wipe each cap gently with a damp paper towel and trim the stem tips. Avoid soaking them, since extra water makes them soggy.
2. Heat the oven. Preheat the oven to 400 degrees F.
3. Make the herb butter. In a medium bowl, mix the softened butter with the green onion, parsley, thyme, salt, and pepper until evenly combined.
4. Arrange the mushrooms. Place the mushrooms gill side up on a large baking tray in a single layer.
5. Spread the butter. Smear the herb butter over the gills and tops, dividing it evenly among the caps.
6. Cover and roast. Cover the tray tightly with foil and bake for about 25 minutes, until the mushrooms are tender.
7. Plate. Arrange the mushrooms on a platter, pour the buttery juices from the tray over them, and scatter with thyme sprigs and extra green onion.
How to Serve Roasted Flat Mushrooms
Serve the mushrooms warm with plenty of the buttery juices spooned over the top. They are a natural partner for slow cooker pot roast and a great way to add vegetables to a holiday plate. Creamy garlic Parmesan orzo and air fryer broccoli with Parmesan make easy sides, and fluffy homemade dinner rolls are ideal for mopping up the juices.
How to Store and Reheat Roasted Flat Mushrooms
Cool leftover mushrooms and their juices within 2 hours, then refrigerate them in an airtight container for up to 3 days. Reheat in a 350 degrees F oven or a skillet until steaming hot. Freezing is not recommended because the texture turns soft and watery.
Tips and Variations for Roasted Flat Mushrooms
Do not soak. Wipe the caps with a damp towel. Soaking adds water that dilutes the butter and softens the texture.
Soften the butter first. Leave the butter out until it spreads easily so you can coat the gills without tearing the mushrooms.
Keep the foil tight. A snug seal traps steam and keeps the herbs green.
Match the sizes. Use caps of similar size so they all finish cooking at the same time.
Save the juices. Pour the butter and mushroom juices from the tray over the caps just before serving.
Add cheese. For a richer dish, sprinkle grated cheese over the caps and uncover the tray for the last few minutes of baking.
Try a vegan version. Swap the butter for plant based margarine for a dairy free option.
Store leftovers safely. Refrigerate promptly in a covered container. University of Minnesota Extension has general advice on storing and reheating leftovers.
Frequently Asked Questions About Roasted Flat Mushrooms
How long do you cook roasted flat mushrooms?
Bake them covered with foil at 400 degrees F for about 25 minutes, until the mushrooms are tender and the butter has melted into a sauce.
Can you make roasted flat mushrooms ahead of time?
Mix the herb butter ahead and keep it in the refrigerator, then bring it to room temperature so it spreads easily. Roast the mushrooms just before serving for the best texture.
What size mushrooms work for roasted flat mushrooms?
Large portobello caps about 4 to 4 1/2 inches wide are ideal, because they hold plenty of butter and cook evenly.
How do I clean portobello mushrooms?
Wipe them with a damp paper towel and trim the stem tips. Do not soak them in water.
Can I use dried thyme?
Yes. Use about 1 teaspoon of dried thyme in place of the fresh thyme leaves.
Roasted Flat Mushrooms with Thyme Butter
Equipment
- Large baking tray
- Aluminum foil
- Mixing bowl
Ingredients
Main Ingredients
- 8 large flat portobello mushrooms about 4 to 4 1/2 inches wide, stem tips trimmed
- 7 tbsp unsalted butter softened
- 1 cup thinly sliced green onion
- ⅓ cup chopped fresh parsley
- 1 tbsp fresh thyme leaves or 1 tsp dried thyme
- ½ tsp kosher salt
- ¼ tsp black pepper
- 3 tbsp chopped green onion for garnish
- 12 fresh thyme sprigs for garnish
Instructions
Cooking
- Preheat. Heat the oven to 400 degrees F.
- Trim and wipe. Trim the stem tips and wipe the mushrooms clean with a damp paper towel.
- Mix the butter. Stir the butter, green onion, parsley, thyme, salt, and pepper together in a bowl until smooth and evenly speckled with herbs.
- Set the tray. Lay the mushrooms gill side up in a single layer on a large rimmed baking tray.
- Coat. Spread the herb butter across the mushrooms, covering the gills and edges.
- Cover. Seal the tray tightly with foil to keep the herbs green.
- Roast. Bake about 25 minutes, until the mushrooms are tender and juicy.
- Finish. Move the mushrooms to a platter, spoon the juices from the tray over them, and top with thyme sprigs and extra green onion.
