To make this baked oatmeal, start by preheating your oven to 350 F.
Next, prepare the wet ingredients: add the eggs, milk, vanilla extract, maple syrup, lemon juice, and lemon zest to a bowl and whisk until they're fully combined.
Next, grease an 8x8 or 9x11 baking dish with oil or butter and set it aside.
Add all of the dry ingredients to the baking dish: the rolled oats, poppy seeds (if using), cinnamon, baking powder, and kosher salt.
Pour the wet ingredients on top of the dry ingredients and mix until they're fully combined.
Then, fold in the blueberries.
Grate additional lemon zest on top of the oat mixture (if desired) and transfer the baking dish to the oven.
Bake the oatmeal for 32-35 minutes, or until the top has set.
Once it's done, remove it from the oven and allow it to cool slightly before slicing it into 8 squares.
Finally, serve and enjoy!