Preheat the oven to 375℉. Prepare an 8x8 casserole dish with nonstick cooking spray.
Mash the bananas. Don't slice the banana for the topping until the oats are ready to cook. Stir the melted vegan butter, vanilla, and ground flax into the mashed bananas and allow to sit while you are preparing the other ingredients.
¾ cup mashed bananas, 2 tablespoons vegan butter, 1 teaspoon vanilla extract, 2 Tablespoons flax meal
Take one cup of the oats and make oat flour by putting the oats into a blender and blending until powdery.
In a bowl, stir together the oat flour, oats, brown sugar, baking powder, cinnamon, nutmeg, and salt.
2 cups old fashioned oats, ¼ cup brown sugar, 1 teaspoon baking powder, 1 teaspoon ground cinnamon, ¼ teaspoon nutmeg, ¼ teaspoon salt
Add the plant milk and the banana mixture to the mixing bowl containing the oat mixture. Stir together. It will look quite runny at this point.
Transfer the batter to the prepared casserole dish, smoothing it into an even layer.
Slice the banana evenly on top of the oatmeal mixture in the baking pan. Sprinkle the brown sugar, chopped nuts, and cinnamon on top of the sliced bananas and oatmeal mixture.
1 large banana, 1 Tablespoons brown sugar, ½ teaspoon cinnamon, 2 Tablespoons chopped pecans
Place the pan into a 375℉ preheated oven. Bake the oatmeal for 45 to 50 minutes. Allow the cooked oats to rest for 5 to 10 minutes prior to slicing and serving.
Serve topped with maple syrup, vegan whipped cream, granola, Greek yogurt or your favorite oatmeal topper.