Trim and quarter strawberries and put in 6" cast-iron skillet along with blueberries. Sprinkle with sugar if using.
Mix all dry topping ingredients together then add butter and work in with fingertips until incorporated and pea-sized.
Spoon topping over fruit pressing down lightly to fit (the berries shrink during cooking and you want lots of crunchy crust). You may have a little left over.
Place on grill rack and put cover down. Check after 10 minutes. Fruit should be bubbly and crust browned - anywhere from 10-20 minutes depending on your grill. Be careful removing - handle will be scorching hot.
Let rest for at least 15 minutes then serve with vanilla ice cream, plain greek yogurt, whipped cream or straight up.