Preheat oven to 350°F. Grease a 12-cup muffin tin or line the cups with paper liners.
In a large bowl, whisk together the dry ingredients: 1 1/2 cups flour, 2 teaspoons baking powder, 1/2 teaspoon baking soda, 1/4 teaspoon paprika, 1/4 teaspoon sea salt, and 1/4 teaspoon black pepper.
In a separate bowl, combine the wet ingredients: 1 cup milk, 1/4 cup plus 2 tablespoons coconut oil (or 1/4 cup plus 2 tablespoons pure butter). Melt the coconut oil or butter and let it cool slightly, then add 1 large beaten egg and the zest of 1 lemon; whisk to combine.
Pour the wet mixture into the dry ingredients and stir gently just until combined—do not overmix.
Fold in 1 cup chopped baby spinach, 1 cup low-fat feta cheese crumbles, and 1 tablespoon finely chopped chives until evenly distributed.
Use a ladle or scoop to divide the batter among the prepared muffin cups, filling each about 2/3 full.
Bake at 350°F for 30 minutes, or until a toothpick inserted into the center of a muffin comes out clean or with a few moist crumbs.
Let the muffins cool in the tin for about 5 minutes, then transfer them to a wire rack to cool slightly before serving.