To avoid lumpy gravy, be sure to sift your flour before toasting if it's lumpy and whisk constantly once you add the roux to the simmering broth. But don't panic if you end up with lumps in your gravy. Just pour it through a mesh strainer, using a spoon or rubber spatula to press it through, and discard any solids (lumps) left behind. Reheat the strained gravy and serve.