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Place a medium heavy-bottom saucepan over medium-high heat and add the butter. Add the onion and bay leaf and saute until the onion is soft, about 5 minutes. Remove the bay leaf. Add the flour and cook, stirring constantly, until the mixture thickens to the consistency of gravy and is creamy and smooth, about 1 minute.
Slowly add the milk, a little at a time, stirring constantly to avoid lumping, until completely combined. Stir in the cream. Keep the mixture at a gentle simmer (not a high boil) until it thickens, 2 to 3 minutes.
Raise the heat to medium. Stir in the mustard and Cheddar and add salt to taste. Simmer for 5 minutes and set aside.
Heat a large heavy-bottom skillet over medium-high heat and add the oil. When it begins to smoke, add the cheese sauce and pasta. Toss thoroughly and continue to cook, stirring occasionally, until approximately 15 percent of the mixture is golden brown, 3 to 5 minutes.
Remove from the heat and serve.