Preheat the oven to 375°F (190°C). Lightly grease an 8 x 8 inch or similar baking dish.
In a large bowl, mash 2 ripe bananas until mostly smooth. Add 1 cup Greek yogurt, ½ cup milk, ¼ cup honey, 1 large egg, and 1 teaspoon vanilla extract. Whisk until creamy.
Stir in 1½ cups rolled oats, 1 teaspoon cinnamon, 1 teaspoon baking powder, and 1 pinch salt. Mix well so no dry oats remain. Gently fold 1 cup blueberries.
Pour into the baking dish. Top with the remaining 1 cup blueberries and ¼ cup walnuts (chopped).
Bake at 375°F (190°C) for about 35 to 40 minutes, until the center is set and the top is lightly golden. Let cool 5 to 10 minutes before serving.
