Heat the oven to 350°F.
Butter a twelve-cup muffin tin or line it with muffin liners.
Blanch or steam the spinach for 2 to 3 minutes until soft. Then drain it, stop the cooking by immersing it in a bowl of ice and water, and then squeeze out as much water as you possibly can.
2 cups packed spinach
Combine the dry ingredients (flour, sugar, baking powder, lemon zest, salt, and cayenne pepper) in a medium to large bowl.
1¾ cups (226 grams) all-purpose flour, 1 tablespoon granulated sugar, 2 teaspoons baking powder, 1 teaspoon lemon zest, ½ teaspoon salt, pinch cayenne pepper
Add the eggs to the measuring cup with the milk and whisk.
1 cup (226 grams) whole milk, 2 eggs
Add the butter and the egg and milk mixture to the dry ingredients.
Stir just enough to wet the dry ingredients, and then add the spinach and crumbled feta and stir until combined.
2 cups packed spinach, 1 stick (8 tablespoons or 4 ounces or 113 grams) unsalted butter, melted, 1 cup (5.3 ounces or 150 grams) crumbled feta cheese
Use a large cookie scoop or large spoon to fill the muffin cups.
Sprinkle the Parmesan on top of the muffin batter.
2 tablespoons (0.5 ounce or 14 grams) grated Parmesan
Bake for 18 to 20 minutes or until a toothpick inserted in the muffin comes out with just a few crumbs.
Switch your oven to broil and broil the muffins for 1 to 2 minutes until the Parmesan starts to brown.
Cool in the pan for 5 to 10 minutes and then remove to a cooling rack. These feta and spinach muffins are definitely best when still a little warm!
Store in an air-tight container in the refrigerator for a few days or in the freezer for up to three months.