This "cheese" sauce has been a favorite of mine for years. It is one of many variations I've seen online and in cookbooks such as The New Farm Cookbook.
It's easy, quick, full of flavor and vitamins, and healthy. To enjoy it, though, you have to get past the idea that it will taste like Velveeta. It doesn't. I happen to hate Velveeta, so that's a good thing for me. The sauce takes its flavor mainly from nutritional yeast, the miracle food I wrote about here. If you've never tried it, now's the time.
In a medium saucepan, combine: 1/2 c. nutritional yeast, 3 T. flour, 2 c. water, 1 t. salt, 1/2 t. garlic powder, 1/4 t. paprika, 1/3 c. vegetable oil, 1 t. yellow mustard.
Heat the mixture over low-medium heat, stirring frequently. As the mixture starts to boil and thicken, whisk a few times more, then remove from the heat.
Here, I put it on top of a baked potato with broccoli: