Cook pasta according to package directions for al dente consistency, drain, reserving 1/2 cup of the pasta water, and set aside.
Set a large pot over medium heat and add the butter. When the butter is melted, whisk in the flour and cook for 1 minute.
Gradually pour in the milk, whisking continuously until smooth. Continue cooking for 1 minute to thicken slightly.
Whisk in the pumpkin purée, mustard, salt, black pepper until incorporated.
Add the cheese and stir until melted. Remove from heat.
Add cooked pasta to the cheese and pumpkin sauce and stir to coat. If the sauce seems thick, add a splash of pasta water to thin to your desired consistency. Adjust seasoning to taste and serve hot.