Bring stock to a simmer, in stockpot, if cold.

Melt butter in a skillet with rounded sides, then add flour to create a roux. Keep stirring/whisking until it turns an ivory color and starts to smell like cookies. Gradually deglaze with some of the stock until it's incorporated and in not danger of clumping, then add deglazed roux to stock. Stir and simmer until the sauce has reduced a bit to end up with a quart of sauce.