Preheat your oven to 375°F (190°C). Grease a 12-cup muffin tin with non-stick cooking spray or line each cup with paper liners to ensure easy removal after baking.
In a large mixing bowl, whisk together the all-purpose flour, baking powder, salt, black pepper, and garlic powder. Ensure there are no large lumps in the flour so the muffins rise evenly.
In a separate medium bowl, beat the egg lightly. Whisk in the milk and olive oil until the mixture is well combined and emulsified.
Pour the wet ingredients into the dry ingredients. Use a spatula to gently fold the mixture together until just combined. Be careful not to overmix, as this can result in tough muffins.
Gently fold the chopped spinach, crumbled feta cheese, and sliced green onions into the batter. Distribute them evenly so every muffin has plenty of flavor.
Divide the batter evenly among the 12 muffin cups, filling each about three-quarters full. Place the tin in the center rack of the oven and bake for 22 to 25 minutes, or until a toothpick inserted into the center comes out clean.
Allow the muffins to cool in the tin for about 5 minutes before transferring them to a wire rack to cool completely. This prevents the bottoms from becoming soggy.