Preparation Preheat oven to 425°F. Toss onions, fennel, and 2 Tbsp. oil on a rimmed baking sheet; season with salt and pepper. Spread out into a single layer. Pinch sausage into small pieces and scatter around onions and fennel. Roast until vegetables and sausage are cooked through and well browned, 25–30 minutes. Meanwhile, cook pasta in a large pot of boiling salted water, stirring occasionally, until al dente. Drain, reserving 1 cup pasta cooking liquid. Combine vegetables, sausage, and pasta in a large bowl. Add lemon zest, lemon juice, remaining 4 Tbsp. oil, and 1/4 cup reserved pasta cooking liquid and toss to coat, adding more pasta cooking liquid if needed, until sauce comes together and coats pasta. Toss in arugula. Divide pasta among plates and top with Parmesan. Season with more salt and pepper.
Preparation Preheat oven to 425°F. Toss onions, fennel, and 2 Tbsp. oil on a rimmed baking sheet; season with salt and pepper. Spread out into a single layer. Pinch sausage into small pieces and scatter around onions and fennel. Roast until vegetables and sausage are cooked through and well browned, 25–30 minutes. Meanwhile, cook pasta in a large pot of boiling salted water, stirring occasionally, until al dente. Drain, reserving 1 cup pasta cooking liquid. Combine vegetables, sausage, and pasta in a large bowl. Add lemon zest, lemon juice, remaining 4 Tbsp. oil, and 1/4 cup reserved pasta cooking liquid and toss to coat, adding more pasta cooking liquid if needed, until sauce comes together and coats pasta. Toss in arugula. Divide pasta among plates and top with Parmesan. Season with more salt and pepper.