01 - Preheat oven to 375°F. Line a 12-cup muffin tin with paper liners or lightly grease with cooking spray.
02 - In a large mixing bowl, whisk together the flour, baking powder, baking soda, salt, and black pepper until well blended.
03 - Place the eggs, milk, olive oil, Dijon mustard, and fresh spinach in a blender or food processor. Blend until the spinach is completely pureed and the mixture turns bright green.
04 - Pour the green spinach mixture into the dry ingredients. Add the shredded cheddar, grated Parmesan, and sliced green onions. Fold gently with a spatula until just combined—be careful not to overmix.
05 - Divide the batter evenly among the prepared muffin cups, filling each about three-quarters full.
06 - Bake for 18 to 22 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
07 - Let the muffins cool in the pan for 5 minutes, then carefully transfer to a wire rack to cool completely.