1. Scrub the lemons under running water. Trim the stem ends, slice thinly, and remove all visible seeds. Reserve the juice and pulp released while slicing.
2. Cover the slices with fresh water in a large saucepan. Bring to a boil and simmer for 5 minutes. Drain, then repeat with fresh water and drain again.
3. Return the slices to the pan. Add the reserved juice and pulp and 4 cups water. Simmer uncovered for 30–40 minutes, or until the peel is very tender.
4. Stir in the sugar until dissolved. Maintain a gentle boil for 25–40 minutes, stirring frequently as the mixture thickens.
5. Test a spoonful on a chilled plate. If it wrinkles lightly when pushed and moves slowly, remove the jam from the heat. If it remains watery, cook a little longer and test again.
6. Leave the jam chunky or briefly blend with an immersion blender, taking care to avoid splashing hot jam.
7. Cool briefly, transfer to clean, heat-safe containers, and refrigerate promptly. Serve chilled or at room temperature in small portions.

Storage: Keep refrigerated at 40°F or below. Because the recipe has not been tested for a validated shelf life, use within 1–2 weeks or freeze for up to 3 months for best quality. This is not a shelf-stable canning recipe.