In a large bowl, combine the strawberries, raspberries, blueberries, 2 tablespoons sugar and mint; set aside. , In a small bowl, beat cream until it begins to thicken. Beat in remaining sugar. Add lemon juice and zest; beat until soft peaks form. Cover and refrigerate until serving., Spread butter over both sides of cake slices. Grill, uncovered, over indirect medium heat for 1-2 minutes on each side or until light golden brown. Serve with berry mixture and whipped cream.