In a large skillet, cook sausage over medium heat until browned and crumbly, about 8 minutes.
Add onions; cook, stirring occasionally, until softened, about 5 minutes.
Stir in kale; cook, stirring occasionally, 2 minutes. Remove from heat.
Add sausage mixture to a 13x9-inch casserole dish.
Stir in toasted English muffin cubes and 1 cup cheese.
In a medium bowl, beat eggs until smooth. Whisk in milk until combined. Stir in salt and pepper.
Pour egg mixture over sausage mixture in casserole dish.
Top with remaining 1 cup cheese. Cover with aluminum foil.
Refrigerate two hours or overnight.
Preheat oven to 350°F.
Bake 20 minutes; uncover, and bake until top is toasted, 15 to 20 minutes more. Garnish with parsley, if desired.