Put 2/3 of the cherries in a pot and roughly blend them with a hand mixer. Make sure not to make them into a purée, you just want to make them into smaller pieces without having to cut them by hand (this will save you a lot of time!).
Add the remaining 1/3 of the cherries and the zest and juice of 1 lemon to the pot.
Cook the cherries, stirring occasionally, until they are completely soft (this will take about 20 minutes).
When cooked, measure out how many cherries you have (juice included) and add 3/4 of that amount in sugar to the pot (e.g. if you have 4 cups of cooked cherries, add 3 cups of sugar).
Stir well and cook over moderate to high heat, stirring often so it does not stick to the bottom of the pan.
While it’s cooking, put a small plate in the freezer.
Once the bubbles subside and the jam appears like it is beginning to gel, put the fire off.
Put a small amount of jam on the frozen plate and return it to the freezer for a few minutes. If it wrinkles when you nudge it, it’s ready.
If not, cook it some more and test it again.
When ready, add the Maraschino, if you want and stir.