Prep the pan and oven: Preheat to 350°F (175°C). Grease a 12-cup mini muffin tin or a 9-cup standard muffin tin, or line with paper liners.
Mix dry ingredients: In a medium bowl, whisk flour, baking powder, baking soda, salt, onion powder, and garlic powder.
Whisk wet ingredients: In a large bowl, whisk eggs, yogurt, milk, and oil until smooth.
Combine: Add the dry ingredients to the wet and stir gently until just combined. The batter will be thick—avoid overmixing.
Add the good stuff: Fold in chopped spinach, shredded cheese, and Parmesan if using.
If using frozen spinach, make sure it’s squeezed dry to avoid soggy muffins.
Fill the cups: Spoon batter into the muffin cups, nearly to the top for mini muffins or about 3/4 full for standard muffins.
Bake: Bake mini muffins for 12–15 minutes or standard muffins for 18–22 minutes, until the tops are set and lightly golden and a toothpick comes out clean.
Cool: Let muffins cool in the pan for 5 minutes, then transfer to a rack. Serve slightly warm or at room temperature.