In a large skillet over medium heat, cook the sausage, breaking it apart as it cooks, and the onion. Drain the excess grease, then add the kale for another minute or two and cook until wilted. Set aside.
While the sausage cooks, crack the eggs into a large bowl. Add the milk, heavy cream, 1 cup of the sharp cheddar, and the salt. Mix well.
Grease a 9×13 glass baking dish with butter and place the sourdough cubes in the dish. Evenly distribute the sausage and onion mixture over the bread cubes. Pour the egg and milk mixture over all of it and mix gently.
Sprinkle the remaining 1 cup of sharp cheddar over the top, cover tightly with aluminum foil, and put it in the fridge for 8-24 hours.
In the morning, preheat the oven to 350 degrees and remove the casserole from the fridge. Bake, covered, for 45 minutes. Remove the tinfoil and bake for another 15-20 minutes.
Allow it to rest for 10 minutes before serving. Enjoy!
