Step 1

Preheat oven to 450 degrees. In a food processor, combine 4 cups spinach, basil, lemon juice, 1/2 cup peas, and olive oil; process until combined. Pulse in cheese; season with salt and pepper. Using kitchen shears, snip sausage into 1-inch pieces. Drizzle with oil; roast on a rimmed baking sheet until undersides are brown, 7 to 8 minutes. Flip and push to one side. Lightly drizzle remaining 3 cups spinach with oil; season with salt and pepper. Scatter evenly over other half of sheet; roast until sausage is cooked through and spinach is crisp, 5 to 6 minutes more.

Step 2

Meanwhile, cook pasta in a pot of salted boiling water according to package directions. Drain, reserving 1 cup pasta water. Return pasta to pot; add pesto, sausage, remaining 1 cup peas, and pasta water, 1/4 cup at a time, until evenly coated. Season with salt and pepper. Serve, topped with crisped spinach, more cheese, and a drizzle of oil.