Adjust oven rack to middle position and preheat oven to 350°F. Lightly grease 2-quart shallow baking dish with butter. Scatter oats, blueberries, ground flax and chia seeds into baking dish.
Whisk 1/3 cup maple syrup, salt, cinnamon, egg, milk, and greek yogurt in a medium bowl until combined. Pour into baking dish. Using a spoon, gently press oat mixture into liquid to moisten evenly.
Scatter brown sugar over the top and bake until center is just set, about 30 minutes. Serve immediately, sprinkling extra brown sugar over before serving.