In a large bowl, combine the ricotta, creme fraiche, egg, and ½ cup of the Pecorino Romano. Season with salt and black pepper and set aside.

Heat the oil in a 12-inch skillet over medium heat. Add the onion and cook until softened, about 4 minutes. Add the garlic, red pepper flakes, and oregano and cook for  2 minutes more. Add the sausage and cook until browned, breaking up the meat with the back of a wooden spoon. Add the tomatoes and bring to a simmer. Season to taste and cook until the sauce has thickened, about 20 minutes.

Preheat the oven to 400° F.

Bring a large pot of water to a boil. Add 2 tablespoons of the salt and return to a rolling boil. Add the pasta and cook for 4 minutes short of al dente according to package directions. Drain, reserving ½ cup pasta water. Add the spinach to the sauce with ½ cup pasta water, stirring until it wilts. Remove from the heat. Stir half of the sauce into the ricotta mixture. Add the pasta and toss together to coat. Pour the pasta into a 13 x 9-inch baking dish. Top with the remaining sauce. Sprinkle the mozzarella and remaining ½ cup Pecorino Romano over the top and bake until the cheese starts bubbling, about 20 minutes.

Set the oven to broil and broil until the cheese and pasta are browned in spots, about 5 minutes more. Remove from the oven and let rest for 5 minutes before serving.