Preheat the oven: Preheat your oven to 350°F (175°C). Grease an 8×8-inch baking dish or line it with parchment paper for easy removal.

1. Make the cinnamon swirl: In a small bowl, combine the brown sugar and ground cinnamon. Stir in the melted butter (or coconut oil) until everything is mixed into a paste-like consistency. Set aside.
2. Prepare the oatmeal mixture: In a large bowl, combine the rolled oats, baking powder, salt, and cinnamon. Stir to combine. Add the milk, eggs, maple syrup (or honey), vanilla extract, and Greek yogurt. Mix well until everything is fully incorporated.
3. Assemble the oatmeal: Pour the oatmeal mixture into the prepared baking dish and spread it evenly.
4. Add the cinnamon swirl: Spoon the cinnamon-sugar mixture over the top of the oatmeal in several spots. Use a knife or skewer to gently swirl the cinnamon mixture into the oatmeal, creating a marbled effect.
5. Bake: Place the baking dish in the oven and bake for 30-35 minutes, or until the top is golden brown and the oatmeal has set. You can test the center with a toothpick to ensure it’s fully cooked.
6. Make the glaze (optional): While the oatmeal is baking, whisk together the powdered sugar, milk, and vanilla extract to make the glaze. Drizzle it over the baked oatmeal once it’s done baking and has cooled slightly.
7. Serve: Let the baked oatmeal cool for a few minutes before cutting it into squares. Serve warm, optionally with fresh fruit, extra syrup, or more nuts on top.