Mash the bananas in a mixing bowl until smooth. A few small chunks are fine but aim for a mostly smooth texture.
Add the eggs, Greek yogurt, milk, and vanilla extract. Whisk until everything is completely combined and creamy.
Add the rolled oats, protein powder, cinnamon, baking powder, and salt. Mix gently with a spatula until evenly incorporated.
Fold in any optional mix-ins such as blueberries, walnuts, chocolate chips, or peanut butter.
Grease an 8×8-inch baking dish and pour in the mixture. Spread evenly with a spatula. Add banana slices on top if desired.
Bake at 350°F (175°C) for 28–35 minutes, or until the top is lightly golden and the center is set.
Let cool for 10–15 minutes before slicing. Serve warm, room-temperature, or chilled.
Store leftovers in the fridge for 5–6 days or freeze slices individually for up to 2 months.