Mix flour, oats, sugar, brown sugar, cinnamon, salt and cardamom in medium bowl. Add melted butter and rub in with fingertips until mixture comes together in small clumps. This topping can be made 1 day ahead--just cover and refrigerate.
Position rack in center of oven and preheat to 375 degrees. Butter a 11x7x2 inch glass baking dish. Combine nectarines, raspberries, 1/4 cup sugar and cornstarch in large bowl; toss to blend. Transfer mixture to baking pan. Sprinkle topping evenly over fruit. Bake until fruit mixture is bubbling thickly and topping is golden brown, about 50 minutes. Cool 20 minutes. Serve warm with vanilla ice cream.