Preheat the waffle iron. You want it fully hot so the waffles crisp up.
Lightly grease it just before cooking.
Whisk the dry ingredients. In a large bowl, combine whole wheat flour, oat flour, baking powder, baking soda, salt, and ground flaxseed. Stir well to distribute the leavening.
Blend the wet ingredients. In another bowl, mash the banana. Add eggs, peanut butter, milk, Greek yogurt, maple syrup, vanilla, and oil.
Whisk until smooth. A few small banana bits are fine.
Bring it together. Pour the wet mixture into the dry. Stir with a spatula until just combined.
The batter should be thick but pourable. If it seems too thick, add 1–2 tablespoons of milk.
Add mix-ins if you like. A handful of mini chocolate chips or chopped peanuts adds crunch and flavor. Don’t overmix.
Cook the waffles. Grease the hot waffle iron.
Add enough batter to cover the grates without overflowing. Close and cook until deeply golden and crisp, usually 3–5 minutes depending on your iron.
Keep them crisp. Place cooked waffles on a wire rack (not a plate) so steam doesn’t soften them. You can hold them in a 200°F (95°C) oven while you finish the batch.
Serve with purpose. Top with sliced banana, a drizzle of maple syrup, a spoonful of Greek yogurt, or a sprinkle of chopped nuts.
For a pre-workout bite, try a smear of extra peanut butter and fresh berries.