Heat the milk in a saucepan.
Melt the butter in a medium pot. Add the onion and saute until translucent. Stir in the flour and salt and cook to thicken.
Add the milk, whisking to blend. Bring the mixture to a steady simmer.
Add the uncooked pasta, cover, and simmer gently for about 5 minutes, until the pasta is only partially cooked. Stir occasionally.
Remove the pot from the heat, stir in the cheese, and pour the mixture into hotel pans to cool.
For each serving, to order: reheat 1 1/4 cups pasta and sauce with a splash of milk if necessary to thin the sauce.