Wash the lemons under cold water, then slice them thinly and remove any seeds.

In a large pot, combine the sliced lemons, granulated sugar, and water. Bring to a boil over medium heat while stirring frequently.

Lower the heat to maintain a gentle simmer for about 40 minutes, stirring occasionally, until it thickens.

Test the consistency by dropping a small amount onto a cold plate; if it holds its shape, it's done.

Let the jam cool slightly, then transfer to sterilized jars and seal tightly.