Add your berries and lemon juice to an enamel or stainless steel sauce pan and cook them over a medium flame.
Just as you begin cooking, mash them with your potato masher, making sure to leave some berries chunkily mashed. Allow to cook for about 3 minutes until bubbling.
Add the sugar, stir and increase to a high flame. Bring to a bubbling boil. Once the berries are boiling, reduce to a medium simmer for about 5 minutes. Continue to stir.
If the jam still appears to be very liquidy after that 5 minutes of simmering and stirring, add cornstarch. The sourness of your berries will be a factor in the consistency of your jam. If they’re sour, it may not require cornstarch, if they’re sweet, they may require up to 2 tsp. If you’ve added cornstarch, give it a nice stir and let it cook for another two minutes on a simmer. Once the consistency is a syrupy thickness, it’s done.
Remove from heat and allow to cool in a container to room temperature. Then cover and refrigerate.