Mix the ingredients: Add the oats, milk, applesauce, walnuts, eggs, maple syrup, vanilla, cinnamon, baking powder and salt to a large bowl. Mix well.
Layer in a casserole dish: Lightly grease a 7x11 inch rectangular baking dish. Spread half the oat mixture in the bottom of the dish. Sprinkle with about ¾ of the blueberries. Top with the remaining oat mixture, carefully smoothing the top. Sprinkle on the remaining blueberries. You can also sprinkle on some extra walnuts if you like!
Chill the dish: Cover the casserole dish and chill in the refrigerator overnight (see notes for instructions to bake right away)
Bake the dish: The next morning, remove the dish from the fridge. Preheat the oven to 350°F. Bake until set and golden on top, about 30-35 minutes. Rest for 5 minutes on a rack on the counter, then slice and serve warm.
Eggs: Instead of whole eggs, you can also use 3 large egg whites. I have never tried making a vegan version without eggs, so I can't recommend any substitutions here.Sweetener: Use a Stevia blend to sweeten the oatmeal if you want to make it sugar-free.
No grey oatmeal: I usually don’t stir the blueberries into the oats, but but sprinkle them on top, to keep the oatmeal from turning all weirdly grey.
Bake without overnight chill
Easy: Make sure to use quick-cooking oats, let the assembled oats sit on the counter for 10 minutes while you preheat the oven, and live with the fact that it’s a little less cake-like 😉