Prep the oven and pan: Heat oven to 350°F (175°C).
Lightly grease a 9-inch square baking dish or similar.
Prep the peaches: In a large bowl, combine 5–6 cups sliced peaches with 2 tablespoons lemon juice, 2 tablespoons brown sugar, 1 teaspoon cinnamon, 1 teaspoon vanilla, a pinch of salt, and 1–1.5 tablespoons cornstarch. Toss until glossy.
Make the protein topping: In another bowl, whisk 3/4 cup flour, 1/2 cup rolled oats, 1/2 cup vanilla protein powder, 1.5 teaspoons baking powder, and 1/4 teaspoon salt. Stir in 1/2 cup Greek yogurt, 1/2 cup milk, and 2 tablespoons melted butter until just combined.
The batter should be thick but spreadable—add a splash of milk if needed.
Assemble: Spread the peach mixture evenly in the dish. Dollop spoonfuls of the topping over the peaches, then gently spread to mostly cover. Leave a few gaps to let steam escape.
Optional flair: Sprinkle with a little cinnamon or coarse sugar for sparkle.
Add 2 tablespoons chopped pecans if you like crunch.
Bake: Bake 30–40 minutes, until the topping is set and lightly golden and the peach juices are bubbling around the edges.
Rest: Let it cool 10–15 minutes. This helps the juices thicken and the topping settle.
Serve: Enjoy warm as is, or top with a spoonful of high-protein vanilla yogurt or a small scoop of light ice cream.