Place strawberries in a medium pot and add sugar and lemon juice.
Mash the berries (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. If you do, you could risk burning it.
Let it cook for 5-6 minutes, stirring the hot preserves every minute.
Pour into a large bowl and let it cool for 30-45 min at room temperature.
Once cooled, transfer the fruit preserves to a Mason jar or other airtight container.