How to substitute flour with nutritional yeast I am planning on making a pretty standard (read: whichever pops up first in Google) recipe for Broccoli Cheddar soup; i.e. brocolli, cheddar, chicken stock, and about 1/2 cup flour per 4 bowls of soup.

To try to reduce using that much bleached flour, I would like to substitute nutritional yeast. Obviously the flavor will be sufficiently similar without taking away from the flavor of broccoli and cheddar (and the stock is pretty potent so I'm also not worried about that) so I am not worried about using too much nutritional yeast.

To achieve the effect of 1/2 cup flour thickening, what is an approximate ratio for the same effect with nutritional yeast? Also, I have noticed with other dishes that, unlikely the grainy/clotty results of flour, nutritional yeast can be added after the fact without whisking. But in a case like this, am I still better off whisking it in at the beginning?
I think this is not such a good idea. It is going to take a lot of nutritional yeast to get anything like the thickening power of flour. If you really want to thicken this with less flour, there are plenty of other things that will do the trick, like xanthan gum, potato starch, corn starch, ultra-tex, some formulations of methylcellulose, etc. Just add however much nutritional yeast you want for flavor or health reasons, and get your thickening elsewhere.