Preheat your oven to 375°F (190°C). Grease a 9x9-inch baking dish or similar-sized casserole dish.

In a large mixing bowl, combine the rolled oats, baking powder, cinnamon, and salt. Stir to mix evenly.

In another bowl, whisk together the milk, maple syrup or honey, egg, melted butter or coconut oil, vanilla extract, lemon zest, and lemon juice until well combined.

Pour the wet ingredients into the dry ingredients and mix until fully incorporated.

Gently fold in the blueberries, being careful not to crush them.

Transfer the mixture to the greased baking dish and spread it out evenly.

Bake for 35-40 minutes, or until the top is golden brown and the oatmeal is set.

Let the baked oatmeal cool for 5-10 minutes before serving. You can enjoy it warm or at room temperature.