Preheat your oven to 375°F (190°C) and prepare your muffin tin with paper liners or a light coating of oil.

In a large bowl, combine the all-purpose flour, whole wheat flour, baking powder, salt, and black pepper. Whisk until well mixed.

In another bowl, whisk together the milk, olive oil, and eggs until well combined.

Pour the wet ingredients into the dry ingredients and stir until just combined.

Gently fold in the chopped spinach, feta cheese, and sun-dried tomatoes with a spatula.

Divide the batter among muffin cups, filling each about 2/3 full. Top with grated Parmesan if desired.

Place the muffins in the preheated oven and bake for 18-20 minutes or until a toothpick inserted in the center comes out clean.

Allow muffins to cool in the pan for a few minutes before transferring them to a wire rack to cool completely.