Preheat your oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners or lightly grease with oil spray.
Make the crumble: In a small bowl, mix oats, almond flour, brown sugar, cinnamon, and a pinch of salt.
Cut in the cold butter with a fork until coarse, sandy clumps form. Chill in the fridge while you prep the batter.
Whisk dry ingredients: In a large bowl, whisk flour, protein powder, baking powder, baking soda, salt, cinnamon, and nutmeg until no lumps remain.
Mix wet ingredients: In a separate bowl, whisk eggs, Greek yogurt, milk, brown sugar, maple syrup, oil, and vanilla until smooth. Stir in lemon juice if using.
Combine: Pour the wet mixture into the dry.
Use a spatula to fold until just combined. Do not overmix—some small streaks of flour are fine.
Fold in peaches: Gently fold in the diced peaches. If they’re very juicy, pat them dry first to avoid soggy muffins.
Fill the tin: Divide the batter evenly among the 12 muffin cups.
Sprinkle the chilled crumble over the tops, pressing lightly so it adheres.
Bake: Place on the center rack and bake for 18–22 minutes, or until a toothpick comes out with a few moist crumbs. Protein bakes can go from perfect to dry fast, so start checking at 17 minutes.
Cool: Let the muffins cool in the pan for 5 minutes, then transfer to a wire rack to finish cooling. This helps set the crumb and prevents sogginess.
Optional glaze: For a bakery finish, whisk 1/4 cup powdered sugar with 1–2 teaspoons milk and a drop of vanilla.
Drizzle over cooled muffins.