Ive done as pp suggested, with plain flour, in the drained pan, then add water. I also add a tsp vegemite, and sauces, such as soy, tomato, chilli etc, depending on what flavour i need.
A tip for lump free, using gravy powder. Add tblspn or two of gravy powder to one cup of water, and stir quickly with a fork. it should all dissolve it the cold water.
then, add it to pan juices ( fat drained)
I use to do lumpy gravy, my dsd still ask for it
but now, i have lump free every time. I cant stand gravy powder on its own, so I still add the extras like vegemite, sweet chilli sauce etc. Extra stock powder adds more flavour too.
Oh, when im in a rush, or dont have pan juices, you can do it in microwave. Mix gravy and water, add extras, stir and pop in MV for 2 mins, stirring a couple of times in between