Preheat and Prepare: First, preheat your oven to 190°C (170°C fan). Lightly grease an 8x8 inch (20x20 cm) or similar-sized baking dish with butter or cooking spray.

Combine Dry Ingredients: In a large mixing bowl, add the rolled oats, baking powder, ground cinnamon, nutmeg, and salt. Give it a good whisk to combine everything evenly. This ensures you don't get a clump of baking powder in one bite!

Combine Wet Ingredients: In a separate medium bowl, whisk the two large eggs. Then, add the milk, maple syrup, melted butter, and vanilla extract. Whisk again until everything is well incorporated and smooth.

Mix Wet and Dry: Pour the wet ingredients into the large bowl with the dry ingredients. Stir with a spatula or wooden spoon until just combined. What works best for me is to stir until there are no dry spots of oats left, but don't overdo it.

Add Blueberries: Gently fold in most of the blueberries, reserving a small handful to scatter on top. If using frozen, add them now, straight from the freezer.

Rest the Batter: Let the mixture sit in the bowl for about 5 minutes. This little pause allows the oats to start absorbing the liquid, which contributes to a better, more even bake.

Pour and Bake: Pour the oatmeal mixture into your prepared baking dish and spread it into an even layer. Scatter the remaining blueberries over the top, along with the optional demerara sugar and sliced almonds if you're using them.

Bake to Perfection: Place the dish in the preheated oven and bake for 35-40 minutes. You'll know it's done when the centre is set (a toothpick inserted should come out mostly clean) and the edges are golden brown and bubbling slightly.

Cool and Serve: Let the baked oatmeal cool in the dish for at least 10 minutes before slicing and serving. This helps it set up properly so you can get clean, neat squares.