Preheat oven to 375 F. Coat an 8-inch-square baking dish with baking spray, or use individual dishes.
Mix together nectarines, 1/2 cup brown sugar, cornstarch, lemon juice and 1/4 teaspoon salt.
Pour into baking dish.
In a medium bowl, stir together flour, oats, 1/3 cup brown sugar, baking powder, 1/4 teaspoon salt, cinnamon and nutmeg. Add 3 tablespoons of butter into the flour by pinching with your fingers until the butter is evenly distributed in small clumps. Sprinkle topping evenly over fruit. Melt remaining butter and drizzle over the topping.
Bake for 30 minutes, until fruit is bubbling and topping is browned. Let cool slightly before serving. Serves 8.