Preheat and Prep: Preheat your oven to 375°F (190°C). Line a standard 12-cup muffin tin with paper liners or brush the cups lightly with olive oil.
Whisk Dry Ingredients: In a large mixing bowl, combine the all-purpose flour, baking powder, baking soda, salt, black pepper, and dried oregano. Whisk thoroughly until perfectly blended.
Whisk Wet Ingredients: In a separate medium-sized bowl, vigorously whisk together the eggs, milk, and extra virgin olive oil until the mixture is fully emulsified and uniform.
Combine to Batter: Pour the wet ingredients into the dry ingredients. Using a rubber spatula, gently fold the mixture together until the flour is just barely moistened. Do not overmix; a few small lumps and dry spots are completely fine.
Fold in the Mix-ins: Add the finely chopped fresh spinach, sun-dried tomatoes, and the majority of the crumbled feta cheese. Fold gently 3 to 4 times just to distribute the ingredients evenly.
Fill the Tin: Divide the batter evenly among the 12 muffin cups, filling each one roughly 3/4 full. Sprinkle the reserved bits of feta cheese and sun-dried tomatoes across the top of each muffin.
Bake to Perfection: Place the tin on the center rack of the oven and bake for 18 to 25 minutes. They are ready when puffed up, golden brown on top, and a toothpick inserted into the thickest part of the center comes out clean.
Cool and Serve: Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to finish cooling. Serve warm or at room temperature.