Sausage and Broccoli Rabe Fettuccine

Bring an 8-quart stockpot of salted water to a boil. Prepare a large mixing bowl with cold water and ice and set aside. Cook broccoli rabe for 3 minutes, drain from the water and plunge into the ice water. Drain when cooled. Season pot with additional salt for pasta and cook fettuccine according to package directions until al dente. Stir well as fettuccini tends to stick together while cooking. Reserve 1 cup pasta water. Meanwhile in a large skillet, heat olive oil over medium-high heat. Cook garlic, onion, fennel seed, chili flakes, sausage. Break up sausage with the back of a wooden spoon. Sauté until sausage is cooked through and turns golden brown, about 5 minutes. Add broccoli rabe and stock and heat through. Drain pasta and add to skillet. Season with 1/2 teaspoon black pepper, and mix in cheeses (tear each mozzarella ball in half before hand). If pasta seems dry, add pasta water a little at a time to create a sauce. Serve immediately.