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 flour, baking powder, baking soda, salt, and black pepper until thoroughly blended.
03 - Combine eggs, milk, olive oil, Dijon mustard, and fresh spinach in a blender or food processor. Blend until spinach is completely pureed and mixture turns bright green.
04 - Pour the spinach puree into the dry ingredients. Add cheddar, Parmesan, and sliced green onions. Fold gently until just combined—batter should remain slightly lumpy.
05 - Divide batter evenly among prepared muffin cups, filling each approximately three-quarters full.
06 - Bake for 18 to 22 minutes until tops are golden and a toothpick inserted into the center comes out clean.
07 - Allow muffins to cool in the pan for 5 minutes before transferring to a wire rack. Serve warm or at room temperature.