1. Preheat your oven to 375 F (190 C) and lightly grease an 8×8 inch baking dish with butter or cooking spray. This prevents sticking and ensures easy removal.

2. In a large mixing bowl, combine the dry ingredients: rolled oats, granulated sugar, baking powder, salt, and lemon zest. Whisk them together until they are well combined, ensuring the lemon zest is evenly distributed for maximum flavor.

3. In a separate medium bowl, whisk together the wet ingredients: milk, egg, melted butter (or coconut oil), and vanilla extract. Whisk until the egg is fully incorporated and the mixture is smooth.

4. Pour the wet ingredients into the bowl with the dry ingredients. Stir gently with a spoon or spatula until just combined. Be careful not to overmix, as this can lead to a tougher texture.

5. Gently fold in the blueberries. If using frozen blueberries, there’s no need to thaw them first. Distribute them evenly throughout the mixture.

6. Pour the oatmeal mixture into the prepared 8×8 inch baking dish, spreading it evenly with your spatula.

7. Bake for 30 minutes, or until the top is lightly golden brown and the oatmeal is set in the center. A good visual cue is when a knife inserted into the center comes out mostly clean.

8. Remove from the oven and let it cool for a few minutes before serving. This allows the oatmeal to firm up slightly, making it easier to serve.