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