Thaw boysenberries or raspberries slightly if frozen.
Wash strawberries well and then pull off their stems and slice
Plug in electric skillet and heat to 400°F.
Put sliced strawberries and boysenberries or raspberries in large bowl.
Mash the berries using a pastry blender or potato masher.
Put 1/4 cup sugar in a small bowl and add the Sure-Jell powder to this sugar and mix well with wooden spoon.
Add the Sure-Jell mixture to the mashed fruit; and stir well
Add butter to fruit mixture. The butter prevents foaming.
Pour fruit mixture into electric skillet and begin cooking. Bring mixture to a full rolling boil, in which the boiling doesn't stop when mixture is stirred; stir constantly.
Quickly stir in the rest of the sugar. Return mixture to full rolling boil and continue to boil, stirring constantly, for a few more minutes. Turn skillet off.
Ladle jam into jars. Make labels, if desired. Keep all jars in refrigerator.