In a large mixing bowl, whisk together 1 1/2 cups unsweetened plain oat milk, 1 large egg, 1/4 cup pure maple syrup, and 1 teaspoon pure vanilla extract until blended.

Add 2 cups rolled oats and 1 teaspoon baking powder to the wet mixture and stir until combined.

Gently fold in 2 cups berries (fresh or frozen; blueberries, diced strawberries, raspberries, blackberries and/or marionberries).

Pour the mixture into an 8"x8" baking pan and spread evenly. Cover the pan with foil or plastic wrap.

Refrigerate the covered pan overnight (about 8–12 hours).

When ready to bake, preheat the oven to 350°F (175°C). Remove the pan from the refrigerator and remove the cover.

Bake uncovered at 350°F until the oatmeal is set and no longer jiggles in the center, about 25 minutes.

Remove from the oven and let cool 5–10 minutes. Sprinkle with the reserved 1/2 cup assorted fresh berries and dust with 2 tablespoons powdered sugar, if desired.

Cut into squares and serve. Pour cream, half and half, or additional milk over individual servings if desired.