Prep your pan and oven. Heat the oven to 350°F (175°C).
Line a 12-cup muffin tin with paper liners, or grease it lightly.
Mix the dry ingredients. In a large bowl, whisk flour, oats, protein powder, baking powder, baking soda, salt, and cinnamon. Break up any lumps, especially in the protein powder.
Whisk the wet ingredients. In a separate bowl, whisk eggs, Greek yogurt, milk, oil, maple syrup, vanilla, and lemon zest (plus juice if using) until smooth.
Combine gently. Pour the wet mixture into the dry ingredients. Stir with a spatula until just combined.
The batter will be thick; don’t overmix.
Add blueberries. Fold in the blueberries gently. If using frozen, toss them in 1 teaspoon flour first to reduce bleeding, then fold in quickly.
Fill the tins. Divide the batter among the 12 cups, filling each about 3/4 full. Sprinkle with coarse sugar or oats if you like.
Bake. Bake for 18–22 minutes, or until the tops are set and lightly golden and a toothpick comes out with a few moist crumbs.
Cool. Let muffins cool in the pan for 5 minutes, then move to a wire rack to finish cooling.
This helps them set and prevents soggy bottoms.