Step 1

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.

Step 2

Meanwhile, cook pasta in a large pot of boiling salted water, stirring occasionally, until al dente. Drain, reserving 1 cup pasta cooking liquid.

Step 3

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.

Step 4

Divide pasta among plates and top with Parmesan. Season with more salt and pepper.