A link to %this page% was e-mailed
For the sauce, melt butter in a saucepan which will be large enough to accommodate the pound of cooked pasta, and saute onion until it becomes translucent. Mix in the flour to make a roux. Gradually add milk and allow thickening over low heat. Remove from heat and whisk in Parmesan. Immediately cover surface of sauce with a sheet of plastic wrap and set aside briefly.