Wash and prepare the strawberries (remove the green tops)
In a medium saucepan, add the prepared strawberries, granulated sugar, butter, and fresh lemon juice. Mix over medium-low heat until sugar dissolves (about 5 min).
Increase the heat and bring mixture to a boil. Stir and mash the raspberries frequently and continue to boil until the jam has thickened (or until it reaches a temperature of 220°F/104°C) This will take approximately 10-15 minutes.
Pour the jam into a clean, tempered jar (at this point you can use a strainer to remove the seeds if you'd like). Allow to cool. Once cooled, cover and refrigerate.
How can you tell if the jam is done without a sugar thermometer?
Place a tempered dish in the freezer for around 15 minutes (be sure it’s tempered if using glass, porcelain or pottery). Take it out and drop a spoonful of the jam onto the plate (take the jam off the heat while you do this).
Push into the jam with your finger, you are looking for it to wrinkle and not flow back into the depression made with your finger.
If the mixture is still quite fluid, put the pan back onto the heat and simmer for five-minute intervals until done.