Heat the oven to 350°F and butter a 9-by-13-inch dish.
Brown the sausage in a skillet, breaking it into small crumbles. Drain excess fat, add the onion and kale and cook for 6 to 8 minutes until the kale is tender and the pan is dry. Cool slightly.
Whisk the eggs, milk, salt, pepper and red pepper flakes. Stir in the sausage mixture and 1 cup mozzarella.
Pour into the dish and top with the remaining mozzarella.
Bake for 35 to 40 minutes, until the middle reaches 160°F. Rest 10 minutes before cutting.