Wash and thoroughly dry the lemons with hot water, scrubbing to remove any wax or residue.

Using a sharp knife or vegetable peeler, thinly peel the yellow zest from the lemons, avoiding the bitter white pith. Cut the zest into fine strips.

Slice off the remaining peel and pith. Segment the lemons and remove seeds. Chop the lemon flesh into small pieces.

In a large saucepan, combine the lemon strips, chopped flesh, and water. Bring to a gentle boil, then reduce heat and simmer uncovered for 15 minutes, stirring occasionally.

Add the granulated sugar and vanilla extract (if using) to the pan. Stir until the sugar dissolves.

Increase heat slightly and simmer for another 30–40 minutes, stirring often, until the mixture thickens to a jam-like consistency. Skim off any foam.

To check if the lemon jam is ready, place a small spoonful on a cold plate. If it wrinkles when pushed, it’s done. If not, continue cooking and test again in a few minutes.

Carefully ladle the hot jam into sterilized jars, seal tightly, and let cool to room temperature. Store in the refrigerator or process further for longer storage.