Whisk flour, baking powder, nutmeg, salt, and black pepper together in a bowl. Mix spinach and onion together in a separate bowl. Add milk and eggs to spinach mixture; mix well. Stir flour mixture into spinach mixture until batter is evenly combined.
Heat about 1 inch of oil in a large, deep frying pan over medium-high heat. Oil is ready to use with a drop of spinach batter added to the oil bubbles immediately.
Drop batter, about 1 tablespoon per fritter, into the hot oil; fry until outside edges of fritters are golden brown, 3 to 4 minutes per side. Remove fritters with a slotted spoon and drain on a paper towel-lined plate. Repeat with remaining batter, adding more oil as needed.
