Meanwhile, in 2-quart saucepan, melt butter over medium-low heat. Stir in garlic; cook 1 minute. Stir in flour, pepper flakes, salt and pepper. Cook over medium heat, stirring constantly, until mixture is smooth and bubbly. Gradually beat in milk with whisk. Heat to boiling, stirring constantly; boil and stir 1 minute. Remove from heat; beat in Parmesan cheese and spinach with whisk, then beat in eggs.