Preheat oven to 350°F. Lightly grease a 9x13 baking dish.
Mash the bananas in a large bowl.
Add the oats, chia seeds, flax, Greek yogurt, milk, eggs, cinnamon, and vanilla. Mix everything together in the same bowl. If you’re adding protein powder, add it now.
I often use a kitchen scale to measure ingredients directly into the bowl to cut down on dishes. Please see ingredient weights above.
Pour into the baking dish and spread evenly. Add your toppings.
Cover with foil and bake for 15 minutes. Remove foil and bake for another 15-20 minutes.
Let cool for 10–15 minutes if eating immediately. Cool completely before storing in the fridge.