In a large skillet, cook the sausage until it's browned. Ad the onion and continue cooking until the onions are translucent. Add the kale and cook until it has been greatly reduced. Transfer the mixture to a baking pan that has been sprayed with avocado oil spray and set aside.
In a mixing bowl, whisk the eggs and heavy whipping cream together, then pour the egg mixture over the sausage, onion and kale mixture.
Layer the top of the casserole with the cheese slices. Or alternatively sprinkle with grated cheese.
Place in the oven and cook for 25-30 minutes, or until the cheese is bubbling and starting to brown.