Cook pasta according to package directions, omitting salt and fat. Reserve 1 cup cooking liquid; drain.
While pasta cooks, heat 1 teaspoon oil in a skillet over medium-high heat. Add walnuts; cook 2 minutes, stirring occasionally. Transfer walnuts to a plate.
Add remaining 4 teaspoons oil and sausage to pan; cook 6 minutes, stirring to crumble sausage. Add garlic, broccoli rabe, and 1/2 cup reserved pasta water; cook 2 minutes, stirring occasionally and scraping pan to loosen browned bits.
Add pasta, remaining 1/2 cup pasta water, walnuts, rind, juice, and red pepper to pan; toss to combine. Sprinkle with cheese. Serve immediately.