Boil water and cook the pasta al dente; save about 1 cup of pasta water before draining
While the pasta cooks, make a roux in a Dutch oven by adding the butter and melting. Wait for the butter to brown slightly, then add the flour and cook for 30 seconds to 1 minute. Slowly add milk a little at a time to make the bechamel sauce, whisking constantly to combine
Once all the milk is added, add the cheese, thyme, and pumpkin (about 3/4 of the can or so) and stir to combine. Add salt and pepper to taste, and use the reserved pasta water to thin out the sauce as needed
Add the cooked pasta to the sauce, stir to combine, and serve