1. Wash and hull the strawberries and cut them into quarters. If some are larger or smaller, just make sure the pieces all about the same size.
2. Toss them in a large pot, such as a Dutch oven casserole made of a nonreactive material, with the sugar. Cut the lemon in half, squeeze the juice into the pot, and add the lemon halves to the berries. Stir well, cover, and let sit a couple of hours (up to 8 hours) at room temperature, stirring one or two times while they marinate.
3. Put a small plate in the freezer. Turn the heat on under the strawberries to medium-high and cook the strawberries, stirring occasionally, as they reduce and the juices thicken. If a lot of foam rises to the surface, skim it away. When the syrup is the consistency of warm maple syrup – it will take about 10 to 15 minutes or so, depending on the pot and berries, turn off the heat and put a spoonful of the jam on the frozen plate and return the plate to the freezer. Check it in a few minutes; if it wrinkles when you nudge it, it’s done. If not, return the plate to the freezer and cook the jam a little more, testing it again. It may take a few times before you get it to the right consistency.
4. When the strawberry jam is done, remove the lemon and stir in a few drops of kirsch, if using, then ladle the jam into the prepared jars.