Preheat the oven to 375° F. Grease an 8x8-inch baking dish with cooking spray, set aside.
In a medium-sized bowl, whisk together the milk, egg, vanilla extract, and coconut oil. *If using maple syrup instead of brown sugar, add it here.
In a large bowl, mix the oats, baking powder, salt, brown sugar, ground cinnamon, and ground nutmeg.
Add the wet ingredients into the oat mixture and stir to combine. *If using nuts, add them here.
Carefully pour the mixture into the prepared dish. Bake for 35 - 40 minutes until the top of the oatmeal is golden brown and the oatmeal is set. Allow the oatmeal to cool slightly before adding icing.
Make the Cream Cheese Icing: With an electric mixer, beat the softened cream cheese and butter until fluffy. Carefully add the sugar. Beat until combined. Then add the cream or milk, mix until combined.
Garnish the whole tray of oatmeal all at once or store the icing in the refrigerator and pipe each serving individually, as desired.