Add a small glass plate to the freezer – you’ll need this a little later to check if the jam is done, and the plate will need to be ice cold.
Add the strawberries to a large, heavy-bottomed stockpot. Mash them to desired consistency (I use a metal potato masher for ease) noting that the strawberries will break down more as they cook.
Over low-medium heat, add lemon juice and sugar and stir until the sugar completely dissolves – you’ll know when it's properly dissolved when there are no more grainy bits on the bottom of the pan, don’t forget to stir from the sides of the pot, too.
Turn up the heat and bring the contents to a full boil. Boil until the mixture thickens and reduces, about 20-30 minutes; gently skimming any white foam that accumulates on the top.
Note that the jam will still be quite loose when it’s hot and will thicken-up significantly as it cools. To test if the jam is ready – add a thin layer of jam (a scant spoonful) to the frozen plate. Return the plate to the freezer for 1 minute, then run your finger through the jam. If the line you’ve created with your finger remains there and the jam doesn’t come back together, it's set. If the jam is not set, continue to boil for another 10 minutes and test again.
Notes
Over-cooking the jam will result in an overly-thickened jam when it cools. Be mindful to check to see if the jam is done at the 20 minute mark and go from there.