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

Make the Sauce:

Heat the oil in your largest skillet, at least 12-inches wide. Add the onion and cook until softened and lightly browned, 6-7 minutes or so. Add the garlic, red pepper flakes, and oregano and cook for 2 minutes more. Add whatever sausage you’re using, and cook until browned, breaking it up into chunks with the back of a wooden spoon.

Preheat the oven to 400°F.

Cook the Ziti:

Bring a large pot of water to a boil, salt generously, and return to a rolling boil. Add the pasta and cook a couple minutes short of al dente. The pasta will continue to cook while baking, so avoid over cooking in the pasta water at this stage. Drain, reserving ¼ cup pasta water.

Finish the Sauce and Assemble:

Add the spinach to the sauce with the ¼ cup of 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 and browning at the edges - about 20-25 minutes. Go longer if you need to make sure everything is hot.

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.