Heat oil in a large heavy-based saucepan or Dutch oven. Add onion, garlic and celery, cook, stirring occasionally, over medium-low heat for 5 minutes.
Add the pumpkin cubes, cook for about 5 minutes, stirring frequently. Add fennel, cook for 3 minutes more.
Turn the heat up to high, add ground beef and break it up a little with a wooden spoon. Turn the heat back down to medium and cook for 10 minutes, stirring occasionally, until the meat is cooked through.
Stir in thyme, sage, marjoram, chili flakes, paprika, salt and pepper. Add beef stock and bay leaf, simmer for 10 minutes. Then add the tomato purée and tomato paste. Cover, simmer for 10 minutes or until the pumpkin is tender.
In the meantime, bring a pot of salted water to a boil and cook the pasta al dente according to the package directions.
Serve plates of pasta with the sauce and ricotta spooned over top. Add a sprinkle of chili flakes and thyme. Enjoy!
