1. Remove lamb from the pan. Set aside to rest. Pour all liquid (from roasting pan) into glass measuring cup or bowl; skim fat off top with spoon; reserving fat.
2. Add enough beef broth to make 3 cups; return defatted juices to pan. Set roasting pan on top burner of stove; bring to boil, scraping sides of pan to get all drippings into juice; reduce to slight simmer.
3. Combine fat and flour in separate pan (you may need to supplement lamb fat with oil or butter). Cook on MEDIUM heat until slightly browned. Remove from heat and cool 1 min.
4. Slowly whisk fat and flour mixture (roux) into hot drippings. Bring mixture to gentle boil, until gravy thickens. Season to taste with salt and pepper; strain.