To make the gravy, use a rubber spatula to scrape the pan juices from the roasting pan into a fat separator, leaving the browned bits in the pan. Let stand 5 minutes; then pour off the dark brown drippings into a 2-quart glass measuring cup; reserve the clear yellow fat in the separator. Add enough stock to the drippings to measure 5 cups.

Place the roasting pan over two burners on high heat. Measure 1/4 cup of the reserved fat and add to the pan. Whisk in the flour (a flat "roux whisk" works best to reach into corners) and let bubble for 30 seconds. Since this is a reduced amount of flour and fat, it won't coat the entire pan so try to concentrate the ingredient to one part of pan to prevent the flour from scorching. One cup at a time, whisk in the stock mixture and bring to a boil. Reduce the heat to medium and simmer, whisking often, scraping up the brown bits on the bottom of the pan, until reduced to about 4 cups, about 3 minutes. For a thicker gravy, whisk in the cornstarch, but keep in mind that as the gravy cools, it will also thicken, so it might not be necessary.

Strain through a coarse wire sieve into a bowl. Taste; season with the salt and, if desired, pepper and rosemary. Transfer to a warmed sauceboat.