Meanwhile, in a large Dutch oven over medium-high heat, add the olive oil. When the oil is hot, add the sausage and cook and crumble with a wooden spoon until no longer pink, about 4 minutes. Add the onion and fennel and cook until softened, about 10 minutes. Add the garlic and red pepper flakes and cook until the garlic is fragrant, about 1 minute. Add the tomatoes and season with salt. Bring to a simmer and add the cream; if the sauce is too thick, add a little water from the pasta. Turn up the heat to medium high and simmer rapidly to thicken the sauce and blend the flavors, about 10 minutes.