Preheat the oven to 325°F.

Put the oats into a small bowl and pour the boiling water over the oats. Cover the bowl and leave the oat mixture to stand for 20 minutes so they swell and thicken.

While the oats are in the hot water soaking, in a large bowl with an electric mixer at medium speed, cream together the shortening, white sugar, and brown sugar. Then mix in the eggs, flour, salt, nutmeg, baking soda, and cinnamon.

Once all the ingredients are combined, uncover your oatmeal mixture and mix all the ingredients together in a bowl.

Transfer the cake batter to a large greased 9 × 13-inch pan and bake in the preheated oven for 40 to 45 minutes.

Start making the frosting while the cake is baking. In a 2 to 2½-quart saucepan, whisk together the flour and sugar until blended and there are no lumps. Continue whisking while slowly adding the milk.

Cook over medium-low heat, whisking almost constantly at first and then constantly as the mixture starts to thicken. Continue cooking until the mixture thickens to a pudding-like consistency. This process can take 25-30 minutes.

To test for doneness, use a wooden spoon to scrape across the bottom of the pan, separating the mixture.

When it takes a couple of seconds for the mixture to fill back in, the mixture is done. It will be very, very thick.

Once the mixture thickens, remove it from the heat and pour it into a glass, heat-resistant bowl to cool. Stir the mixture periodically while it’s cooling.

Once the mixture has cooled, use a stand mixer to cream the margarine, shortening, and vanilla together for about two minutes. Switch to a whisk attachment and add the cooled sugar, flour, and milk mixture. Whisk on high for ten minutes until the frosting has reached a thick, fluffy frosting consistency.

When the cake has cooled, generously spread the oatmeal cake frosting over the top of the cake. All that’s left to do is cut into portions, serve and enjoy.