Pull green tops off strawberries. Slice the strawberries and put half in a small saucepan. Add pineapple chunks, jam and lemon juice. Bring to a simmer over medium heat and cook, stirring occasionally, until the strawberries are soft, about 5 minutes. Carefully stir in the remaining strawberries (the sauce will be very hot). Set aside to cool.
To serve, spoon most of the sauce into 4 ice cream dishes and top with scoops of frozen yogurt. Spoon the remaining sauce on top. Garnish with mint sprigs if desired.