Place blueberries in a medium pot.
Add sugar and lemon juice.
Mash the blueberries and ingredients with potato masher (if they are too hard, wait to mash them)
Bring mixture to a boil over high heat, and every so often, mash the berries with the masher.
Once it’s boiling and you’ve mashed the berries, turn them down to medium-high and add the cornstarch slurry (cornstarch plus water).
It will begin to thicken quickly, so do not take your eye off it.
Let it cook for 5-6 minutes, stirring every minute.
Pour into a large bowl and let it cool for 30-45 min at room temperature.
Once cooled, transfer preserves to mason jars or other airtight containers.