In a large mixing bowl, add the dry ingredients: oats, brown sugar, baking powder, and salt. Stir until combined. Set aside.
In another bowl, add the wet ingredients: milk, maple syrup, vanilla, and cooled, melted butter. Whisk until the eggs are beaten.
Add the wet ingredients to the dry ingredients. Stir until combined.
Add the chopped strawberries and rhubarb to the oatmeal mixture. For even baking, make sure the strawberries and rhubarb are cut to a similar size.
Grease a 9x13-inch baking dish with nonstick cooking spray or softened butter.
Pour the oatmeal mixture into the prepared pan. Top with additional cut-up strawberries and rhubarb if wanted.
Bake in a preheated 350 degree F oven for 30-35 minutes or until the edges are golden brown and the center is set.
Allow the baked oatmeal to rest for 5-10 minutes before serving.
Top with whipped cream or vanilla Greek yogurt and a handful of fresh berries.
Notes
Note 1: Do not substitute with quick oats or steel-cut oats. For a gluten free recipe, use certified gluten free oats. Note 2: For a dairy free option, use unsweetened almond milk. Note 3: For the best flavor, use pure maple syrup. Imitation maple syrup often contains high-fructose corn syrup. Note 4: Make sure the melted butter is cool. Otherwise, it will scramble the eggs. For a dairy free option, use coconut oil.