I found the best recipe for any kind of "crisp" topping.
1 cup rolled oats (Quaker Oats)
1/2 cup brown sugar
1/2 cup flour
1/4 c. sliced almonds or walnuts
5 Tablespoons butter, cut into small pieces.
Work together with your fingers to form coarse crumbs. Sprinkle over any fruit in baking dish. 350F for 30-40 minutes till browned. I've done it with apples, pears, and peaches. Today I'm going to try Mangos!