Put a few small plates in the freezer.
Wash and hull the strawberries and add them to a large pot. Mash the berries with a potato masher so that you have a thick pulp. This should yield about 6 cups when mashed.
Add sugar and lemon juice to the mash and stir over medium-high heat until the mixture comes to a boil.
Let jam boil for one minute, then check for how thick the jam is by spooning some onto a plate from the freezer and running your finger through the center of the jam. If the line that your finger made quickly disappears, you will want to cook your jam a bit longer to thicken it further. If the jam stays on either side of the line, your jam is ready.
Remove from heat and skim off the foam. Then ladle jam into clean, sterilized jars. You can also use plastic jars for freezer jam, but ensure that the jam is cooled down before filling them.
Once cooled jam can be stored in the fridge for a few weeks and in the freezer for a year.