Preheat the oven to 375°F and grease an 8x8 inch baking dish with butter or nonstick spray.
In a large bowl, stir together the oats, baking powder, cinnamon and salt.
In a separate bowl, whisk the milk, Greek yogurt, maple syrup, melted butter, ground flaxseed, vanilla and lemon zest until smooth. Pour it over the oat mixture and stir well.Let the mixture sit for 10 minutes. This gives the oats a head start on softening and lets the flaxseed thicken the liquid, which helps the oatmeal set into soft, sliceable squares without eggs.
Fold 1 cup of the blueberries into the oat mixture and spread it evenly in the baking dish. Scatter the remaining blueberries over the top and sprinkle with the brown sugar.
Bake for 35-40 minutes, until the center is set and no longer jiggly and the edges are golden and bubbling.
Let the baked oatmeal rest for 10 minutes to firm up before cutting into squares. Serve warm with a splash of cold milk or an extra drizzle of maple syrup.