Prep the pan and oven. Heat the oven to 350°F (175°C). Line a 12-cup muffin tin with liners or spray well with nonstick spray.
Make oat flour (if needed). Blend 1 cup of rolled oats in a blender until fine. You’ll need about 1 cup of oat flour for this recipe.
Whisk dry ingredients. In a large bowl, combine 1 cup oat flour, 3/4 cup whole wheat (or all-purpose) flour, 1/2 cup vanilla protein powder, 1 1/2 teaspoons baking powder, 1/2 teaspoon baking soda, 1/4 teaspoon salt, and 1/2 teaspoon cinnamon. Stir to blend.
Mix wet ingredients. In a separate bowl, whisk 1 cup plain Greek yogurt, 2 large eggs, 1/3 cup milk, 1/4 cup honey or maple syrup, 3 tablespoons oil, 1 teaspoon vanilla, and 1 teaspoon lemon zest until smooth.
Combine gently. Pour the wet ingredients into the dry. Stir with a spatula until just combined. The batter will be thick—this is good for tall, tender muffins.
Fold in blueberries. Gently fold in 1 to 1 1/4 cups blueberries. If using frozen, add them straight from the freezer to reduce streaking.
Fill and top. Divide the batter evenly among the muffin cups, filling each about 3/4 full. Sprinkle a pinch of coarse sugar on top if you like a slight crunch.
Bake. Bake for 16–20 minutes, until the tops are set and a toothpick comes out with a few moist crumbs (not wet batter). Ovens vary, so start checking at 16 minutes.
Cool. Let muffins cool in the tin for 5 minutes, then transfer to a rack to cool fully. This helps them set and stay moist.