Preheat oven to 375F (190C). Place cookies on a piece of waxed paper and, using a rolling pin, crush into pieces, about 1/4 inch (0.5 cm) in size. You should have about 3/4 cup (175 mL). Place in a medium-size bowl along with flour and brown sugar. Stir. Then cut in butter with 2 knives or your fingers until crumbly and about the size of peas.
In a large bowl, using a fork, stir granulated sugar with cornstarch, cinnamon and nutmeg until blended. Peel mangoes and cut flesh away from pit and into 1-inch (2.5-cm) chunks. You should have about 3 cups (750 mL). Add to bowl and toss. If using fresh rhubarb, cut off and discard leaves before slicing. If using frozen rhubarb, rinse briefly under cold running water to remove ice crystals, then pat dry. Toss rhubarb with mango mixture. Then turn fruit mixture into an 8-inch (20-cm) square or round baking dish that will hold at least 8 cups (2 L).
Sprinkle cookie mixture over fruit but do not press down. Set baking dish on a baking sheet to catch any spills. Bake in centre of 375F (190C) oven until fruit is tender, centre is hot and topping is a deep golden brown, from 45 to 50 minutes. If necessary, cover loosely with foil during last 15 minutes of baking to prevent over-browning. Serve warm with whipped cream or vanilla ice cream. Any leftovers will keep well in the refrigerator, covered, for 2 days. Reheat in microwave or oven.