In large skillet, brown sausage meat over medium high heat, breaking up pieces with the back of a spoon. Add onion and cook until softened, about 5 minutes. Stir in garlic and cook 1 minute. Stir in California walnuts.
Meanwhile, cook pasta according to package instructions. Add rapini during the last minute of cooking. Reserve 1/2 cup (125 mL) of the pasta water in a measuring cup. Drain pasta and rapini in colander. Add reserved pasta water, lemon zest and Parmesan cheese to the walnut mixture. Stir in pasta and rapini. Serve with additional Parmesan cheese and chili flakes if desired.