Cook fettuccine according to package instructions. Drain and reserve 1 cup pasta water.
Meanwhile, heat oil in a saucepan set over medium. Saute onion until softened, about 5 minutes. Add tomato and cook another 3 minutes.
Stir in pumpkin, thyme, and cream. Add reserved pasta water, a little at a time, until you reach desired consistency. Cook sauce until heated through, then add salt and pepper to taste.