Bring a large pot of lightly salted water to a boil.

Heat a large skillet over medium-high heat with a splash of olive oil (use slightly more if using chicken sausage, less if using pork). Remove sausage from casing and add to skillet, breaking up the sausage into smaller pieces as it starts to cook. Cook for 2 to 3 minutes or until mostly cooked through.

Push sausage to the sides of the skillet. To the middle, add onion and garlic and cook until softened and fragrant, 3 to 4 minutes.

Add red pepper flakes and tomato paste and stir for 30 seconds until evenly coated. Add flour and stir until no dry bits of flour remain.

Add white wine and whisk until mostly evaporated. Add chicken broth, a splash or two at a time, whisking constantly until smooth and all of the broth has been incorporated.

Add pasta to pot of boiling water and set the timer for about 1 minute less than the package instructs (the pasta will finish in the sauce).

Continue to whisk sauce mixture over medium heat until it starts to bubble gently and thicken, about 5 minutes, then whisk in cream. Add salt and pepper to taste (if you used unsalted or low sodium chicken broth, you'll probably need 1/2 teaspoon of salt or more; if your broth was on the salty side already use less). Your pasta should be just about done at this point, if not, reduce the heat on the sauce to keep it warm until the pasta is just barely al dente.

Use a large slotted spoon or colander to drain pasta and dump it straight into the saucepan with the sauce, reserving a cup or so of pasta water.

Add spinach and sundried tomatoes, then gently fold to coat the pasta completely with sauce, and let it finish cooking for a minute or two until the sauce is nice and thick and coats the pasta completely and the spinach is gently wilded. Add a splash of pasta water if needed to loosen up the sauce a bit more.

Spoon pasta into serving bowls, top with shaved parmesan cheese; serve immediately.