Cook pasta to al dente. Preheat an oven to 400°F (205°C), then coat a 9x13-inch baking dish with a little oil. Bring a large pot of water to a rapid boil, then season generously with a few tablespoons of Kosher salt. Add 1 lb dried ziti pasta, then cook according to package directions until al dente (typically about 3 minutes less than suggested full cook time). Reserve ½ cup of pasta water, then drain the pasta and set aside. While the water is heating, move onto the next step.
Brown the sausage. Heat 2 Tbsp olive oil in a large skillet or Dutch oven over medium-high heat. When hot, add 1 lb Italian sausage, then use a spatula to break up into smaller pieces. Cook until browned, about 4-6 minutes. Add 1 diced onion, then cook an additional 3-4 minutes, stirring occasionally.
Sauté aromatics. Stir in 2 Tbsp fresh oregano leaves (or 2 tsp dried oregano), 4 cloves grated garlic, ½ tsp crushed red pepper flakes, and ½ tsp fennel seeds, then cook for an additional 1 minute. Add 2 Tbsp tomato paste, then use a spatula to stir it into the onion and sausage.
Add the liquid. Pour in ½ cup red wine to deglaze the pan, then use a spatula to scrape any browned bits off the bottom. Cook until reduced by half, about 2 minutes, then add 1 (28-ounce) can of crushed tomatoes.
Add spinach. Stir in 5-ounces fresh baby spinach (or Swiss chard leaves) until wilted, then season with 1 tsp Kosher salt and ¼ tsp black pepper. Pour in the drained ziti pasta, along with ¼ cup reserved pasta water. Stir well until the sauce evenly coats the pasta.
Make the ricotta mixture. In a large bowl, mix together 15-ounces ricotta, ¾ cup grated Pecorino Romano (reserve the rest), and 2 tsp lemon zest.
Assemble. Pour half of the ziti and sausage mixture into the prepared baking dish, then dollop with half of the ricotta cheese and half of the mozzarella cheese. Repeat with remaining ziti, ricotta, and mozzarella, then sprinkle the top with ¼ cup of grated Pecorino Romano.
Bake, then serve. Transfer to the oven and bake for 20-25 minutes, or until the cheese is bubbling. Adjust the oven temperature to broil, then carefully place the baked ziti on a rack about 4-6-inches from the heat source. Broil on high for a few minutes, until the cheese on top turns golden brown. Keep a close eye on it so the top doesn’t burn! Allow the ziti to rest for 5-10 minutes before serving. Garnish with fresh basil or oregano leaves, then serve immediately!