In a large heavy skillet, cook and break apart/crumble sausage with onions over medium high heat. Once evenly browned, drain and set aside.In a bowl, add eggs and beat lightly. Add salt, garlic powder, pepper, and milk. Whisk together to combine well. Add bread cubes and gently stir to coat well. Add cheese and gently fold to incorporate.Grease a 9x13 baking dish. Pour egg/sausage mixture evenly into dish, ensuring edges are evenly covered as well. Cover tightly with foil and chill overnight.Remove casserole from fridge and let sit at room temp while you preheat oven to 350F with rack on lower middle position. Keeping casserole covered with foil, bake about 50-60 minutes or until center is set. Uncover and sprinkle ½ cup more cheese evenly on top; bake 10-15 minutes more or until top is golden brown. Garnish with chopped parsley if desired.