Cook pasta in a large pot of salted water according to package directions for al dente – reserving 1 1/2 cups of pasta water for sauce before draining.
In a large skillet, brown Italian sausage over medium-high heat, breaking it into pieces as it cooks, about 8 minutes. Transfer cooked sausage to a plate and set aside.
Add olive oil to pan and sauté shallot, garlic and crushed red pepper, stirring frequently, until softened and slightly golden, about 3 minutes.
Add spinach to pan and wilt, stirring occasionally. If needed, reduce heat to low until pasta is ready.
Return heat to medium-high. Add pumpkin puree, sage and salt & pepper. Slowly stir in pasta water, about 1/2 cup at a time, until sauce has reached desired consistency. Return sausage to pan and toss with pasta, adding extra pasta water if needed.