I would not add flour AND cornstarch. Old-fashioned gravy is made with flour. As some have described, add some flour to the fat left in the pan, brown it, then slowly add water while whisking. I prefer to use watered down cream for that step.
If you choose to use cornstarch, as I do - because I am gluten intolerant, as someone mentioned, mix the cornstarch with COLD water, then whisk it into your broth. Bring it to a boil until it thickens.
Either way, if you cook it too long, or on high temp., the fat will separate from the broth and you'll have
a watery mess.