Preheat your oven to 350°F and grease a 9x13 inch baking dish with butter or cooking spray. While the oven heats, measure out all your ingredients and set them nearby—this makes the mixing process smooth and prevents scrambling. I always bring my eggs and milk to room temperature before starting; room temperature eggs emulsify much better with the yogurt, creating a creamier batter.
In a large mixing bowl, whisk together the oatmeal, almond meal, baking powder, baking soda, salt, cinnamon, nutmeg, and sugar until evenly combined. This ensures the leavening agents and spices are distributed throughout the oats, preventing any pockets of baking soda taste or uneven rising during baking.
In a separate bowl, whisk together the room-temperature eggs, Greek yogurt, and vanilla essence until smooth and well combined. Slowly add the cooled melted butter while whisking constantly to incorporate it evenly, then pour in the milk and stir until the mixture is homogeneous. The yogurt adds tanginess and moisture while the eggs act as a binder—this wet base is what transforms the oatmeal into a custard-like bake rather than a dry crumb.
Pour the wet ingredient mixture from Step 3 into the bowl with the dry ingredients from Step 2, and gently fold together using a spatula or wooden spoon until just combined—don't overmix, as this can develop the gluten in the oats and make the bake tough. The batter should look thick but pourable, with streaks of dry ingredients okay if they disappear with one or two more gentle folds.
Gently fold the fresh blueberries into the batter from Step 4, being careful not to crush them and turn the batter blue. Pour the entire mixture into your prepared 9x13 inch baking dish and spread it evenly. Sprinkle the cinnamon sugar topping over the surface—I like to use my fingers to lightly press it into the top layer so it adheres better and creates a nice texture contrast.
Bake in your preheated 350°F oven for 30–40 minutes, until a toothpick inserted in the center comes out clean or with just a few moist crumbs. The top should be lightly golden and set. Remove from the oven and let cool for at least 15 minutes before serving—this resting time allows the custard-like center to set and makes it easier to cut into neat portions.