Boil the pasta in a pot of heavily salted water for 2-3 minutes less than the “Al Dente” instructions on the box. 10 minutes for me.
Meanwhile, heat a Dutch oven or large pot over medium-high heat.
Add the salsiccia and fennel seeds; mix together and brown.
When the pasta is ready, add it to the sausage using a spider.
Add 3 handfuls of spinach and stir everything together.
Add about 4 cups or 1 2/3 jars of pasta sauce, ½ cup of pasta water and ¼ cup mozzarella cheese to the pasta; mix everything together.
If using a Dutch oven, drizzle the remaining sauce over the top of the pasta and sprinkle with the remaining mozzarella cheese. If not using a Dutch oven, pour pasta into a 9x13 pan or casserole dish sprayed with non-stick cooking spray and top with remaining sauce and cheese.
Bake uncovered in the oven for 30 minutes or until cheese is bubbly and slightly browned. Let rest at least 5-10 minutes before serving.
Serve with freshly grated Parmigiano-Reggiano cheese.