Wash and drain the mirabelle plums. Cut them in half and pit them.

Weigh them to get 1 kg of pitted mirabelles. Place them in a bowl with the sugar and mix to thoroughly coat them with the sugar. Add lemon juice if desired.

Cover and let macerate in the refrigerator overnight. Alternatively, you can macerate them for just 2 to 4 hours at room temperature.

The next day, or after 2 to 4 hours, transfer the mixture to a large pot and mix well.

Sterilize the jars by placing them with their lids in a large pot. Cover with boiling water and boil for 15 minutes. Drain on a towel along with the lids.

Bring the pot with the mirabelles and sugar to a boil over high heat, stirring occasionally. Cook for 15 minutes from the boiling point, stirring regularly with a wooden spoon.

Check the jam's texture by mashing the mirabelles more or less with a skimmer. Skim off any foam toward the end of cooking.

The jam is ready when it starts to drop from the wooden spoon or skimmer in drops.

To test if the jam has set, place a small spoonful on a plate taken from the freezer. If the jam sets and doesn’t run when you tilt the plate, it’s ready.

If using a thermometer, remove from heat at 104/105°C (219/221°F).

For smooth mirabelle jam, blend with an immersion blender.

Fill the jars with the hot jam almost to the top, using a funnel to avoid spills. Seal with the lids, tightening them well. Turn the jars upside down on a towel.

Allow to cool completely before turning them back to their upright position. Store in a cool, dark place. I keep mine in the cupboard.

The jam will keep for a year or more this way.