Skim
the fat off of the drippings from the roast. (Fat
will ruin the texture of your gravy.)
Pour 1/2 cup
of the chicken broth into the roasting pan with the
skimmed drippings, and stir it around, scraping up
the yummy browned bits from the rack and the bottom
of the pan. When most of the stuck-on stuff is
dissolved into the broth, put the roasting pan over
medium-high heat.
Put the rest of the chicken broth
in a blender with the guar or xanthan, and run the
blender for a few seconds to dissolve all of the
thickener.
Pour the thickened broth into the
roasting pan, and stir until all the gravy is
thickened. (It it gets too thick, add a little more
chicken broth; if it's not Quite thick enough, let
it simmer for a few minutes to cook down.)
Salt and
pepper the gravy to taste, and serve with the leg of
lamb.