In a large mixing bowl, mash 3 cups cooked sweet potatoes and mix with ½ cup sugar, 2 eggs, 1 stick melted butter or margarine, 1 teaspoon vanilla, and 1 teaspoon ground nutmeg. (You may use an electric mixer to make a smooth mixture.) Pour into a greased 8-inch by 10-inch (or similar) oven-proof baking dish. (I used cooking oil spray to grease the baking dish.) Now make the topping: Mix ½ cup flour, ½ stick cool butter or margarine, 1 cup light brown sugar, and 1 cup chopped pecans, until crumbly. (You may want to use a pastry blender for the flour, butter, and brown sugar, and then stir in the pecans afterward.) Sprinkle topping over the sweet potato mixture in the baking dish, and bake at 350 degrees until bubbly and beginning to brown on top (about 20 to 30 minutes). This is my favorite holiday sweet potato casserole. It is very easy to make and the flavor is terrific! Enjoy!!!