In a large skillet, melt butter and olive oil until the butter is foamy. Add the diced onion and cook for 3-4 minutes, until softened. Add the minced garlic and cook for 1 minute. Add the kale and cook, stirring occasionally until the kale is wilted. Add the cream and reserved pasta water and bring to a simmer. Season with a little salt and pepper and the crushed red pepper. Add the pasta back to the pan and toss with the sauce. Lower the heat and add all four cheeses, reserving 1 cup of the cheddar and fontina mixed. Stir until the cheese is melted. Top with the reserved cheese and place under the broiler on high for 4-5 minutes, or until the cheese is bubbly and browned.