Bring a pot of water to boil and add the spinach. Cook for 2 to 3 minutes. Transfer to a bowl of ice water until cool. Drain.
Put in a dry bowl and add eggs, grated cheese, milk, and flour.
Season with nutmeg, garlic powder, salt, and pepper.
Add enough oil to coat the bottom of a large skillet set over medium heat. Carefully spoon the batter into the skillet. Cook the croquettes on both sides until browned.
Remove the croquettes from the skillet and place them on paper towels to remove excess oil. Continue, adding more oil as needed, until all croquettes are cooked.