Boil the rigatoni until tender, about 8 minutes. While the pasta cooks, place the butter and oil in a very large skillet set over medium heat. When the butter is melted, add the garlic and chili flakes and cook until fragrant, about 1 minute. Add the stock and bring to a simmer. Swirl in the spinach and cook until it just wilts. When the pasta is cooked, drain well, reserving 1/2 cup of its cooking liquid. Add the rigatoni, reserved cooking liquid, and salt to the skillet and toss to combine. Divide the pasta among bowls, sprinkle with cheese and serve.