Put plenty of water and about 1 tablespoon of salt (not listed in the ingredients) in a bowl, and wash the strawberries.
Drain off the water, and cut off the tough hull parts.
Gently wrap each berry in paper towels to blot off any water. If you aren't using the berries whole, cut them at this stage into 1/2 or 1/4. Weigh the strawberries at this point.
Put half the strawberries in an enamel pan (or another kind of pan if you don't have an enameled one), and cover evenly with about 1/4 of the sugar. Layer in the rest of the strawberries and sugar.
Cover and leave to marinate for 6 to 8 hours (or for a shorter time if you cut the berries). When the granulated sugar has almost completely melted, they're ready.
Start cooking on low heat. When bubbles rise up to the surface, stir with a wooden spatula and simmer over medium heat.
When scum rises to the surface, skim it off completely. Adjust the heat so that the jam doesn't boil over, and simmer for about 15 minutes.
When the strawberries are soft and the jam has thickened a bit, turn the heat off, add the lemon juice, stir, and it's done.
If you crush the berries at the end, the jam will thicken.