Bring a large pot of salted water to a boil. Add the pasta and cook as the label directs. Reserve 1/2 cup cooking water, then drain.
Meanwhile, pulse 2 cups spinach, 2 tablespoons water, 1 tablespoon each olive oil and nuts, the lemon zest and juice, 1/2 teaspoon salt and the red pepper flakes in a food processor until smooth. Scrape down the sides; add the parmesan and pulse until smooth. Set the pesto aside.
Form the sausage into nickel-size patties, about 1/2 inch thick. Heat the remaining 1 tablespoon olive oil in a large nonstick skillet over medium heat. Add the patties and cook until browned, about 3 minutes. Flip, add the garlic to the skillet and cook until the patties are just cooked through, about 2 more minutes.
Add the pasta, pesto and 1/4 cup reserved cooking water to the skillet. Toss to coat, adding more cooking water as needed; season with salt and pepper. Stir in the remaining 2 cups spinach. Top with parmesan and the remaining 1 tablespoon nuts; drizzle with olive oil.
Photograph by Charles Masters
Courtesy of Food Network Magazine