In a bowl, combine the flour, oatmeal, baking powder, salt, nutmeg and the shortening. Cream the shortening with the sugar and eggs.
Add the dry ingredients to the wet alternating with the banana and buttermilk. Spread into a 9 x 11 baking dish.
Bake in a 350 oven for 35 - 40 minutes.
Topping: Combine all the ingredients in a small saucepan. After the cake has cooked for 40 minutes, spread the topping over the cake. Broil with the oven door partially open for 2 minutes. Watch carefully as it could burn.