Set a large skillet over medium heat and add the sausage. Cook until browned and cooked through, 7 to 9 minutes. Remove the sausage with a slotted spoon and drain on a paper towel lined plate. Without cleaning the skillet, add the apples, onion and sage and cook until the onion has softened, 2 to 3 minutes. Remove the pan from the heat and set aside.
Mix together the eggs, milk and half-and-half in a large pitcher or bowl. Season with salt and pepper and set aside.
Grease a large casserole dish with some butter. Layer in half the English muffins, half the sausage, half the apple/onion mixture and half the cheese. Repeat with the other half of the same ingredients, ending with the cheese. Slowly pour the egg mixture all over the top. Cover with plastic wrap and refrigerate overnight.
Remove the pan from the fridge 20 to 30 minutes before baking. Preheat the oven to 350 degrees F.
Replace the plastic wrap on top of the casserole with aluminum foil. Bake for 35 to 40 minutes, then remove the foil and continue to bake until the top is golden brown and slightly crisp, 10 to 15 minutes more.
Sprinkle with the chopped parsley. Serve warm and enjoy.