1. Melt the margerine in a pan (old, seasoned cast iron is always the best), and before it browns whisk in the nutritional yeast. You want the mixture to be crumbly, but not dry or too squishy.
2. Add the flour, and if necessary, a little more margarine to maintain texture.
3. When the mixture is slightly browned and aromatic, whisk in enough water to make a fairly thin sauce. It will thicken as it heats.
This is a delicious substitute for gravy over potatoes, cauliflower, broccoli, peas, etc.