Preheat oven to 350°F Grease a 9x13-inch pan.
Combine the flour, oatmeal, baking powder, baking soda, salt and nutmeg. Mix well and set aside.
At medium speed of an electric mixer, cream together the shortening and sugar until light and fluffy. Add the eggs, one at a time, beating well after each addition.
At low speed, add the flour mixture alternately with the buttermilk and mashed bananas, beginning and ending with the flour mixture. Beat only until ingredients are blended; do not overbeat. Stir in the vanilla extract.
Spread evenly in the prepared pan, and bake for 35 to 40 minutes or until a cake tester comes out clean.
Allow cake to cool for 10 minutes.
In a small saucepan, melt the butter. Add the brown sugar, milk, coconut and pecans, and stir well. (Or, melt the butter in the microwave and combine with the remaining ingredients.) Spread the topping evenly over the cake.
Place cake in a preheated broiler about 6 inches from the heat. Broil for 2 minutes or until golden brown.