re: Instead of making a Bechamel, can I use alfredo sauce in the jar

Cook the flour & butter together for at least three minutes after the butter has completely melted & is sizzling. Don't be afraid to cook it until it is golden brown; the color of the cheese will cover up any slight browning of the sauce. You need to cook it for 3-4 minutes or the flour will taste raw in the finished sauce.

Once you add the milk, reduce the heat, stir nearly constantly, and cook until it is as thick as you'd like. If you boil the holy hell out of it, it will thicken then separate and thin out. So do this part over gentler heat.