Step 1: In a wide, heavy-bottomed saucepan, combine the prepared strawberries and granulated sugar. Use a potato masher to lightly crush the fruit to your desired consistency.
Step 2: Let the strawberry and sugar mixture sit for approximately 30 minutes at room temperature to allow the juices to release naturally.
Step 3: Stir in the fresh lemon juice and add the butter if using. Place the pot over medium-low heat and stir until the sugar has completely dissolved.
Step 4: Increase the heat to medium-high and bring the mixture to a full rolling boil. Continue to cook, stirring frequently to prevent burning, for 15-20 minutes or until the jam reaches 220°F (105°C) or passes the wrinkle test on a frozen plate.
Step 5: Remove from heat and skim off any remaining foam. Ladle the hot jam into sterilized glass jars, leaving 1/4 inch of headspace. Wipe the rims, apply lids, and allow to cool completely at room temperature for 24 hours before moving to the refrigerator.