The secret to great gravy is the juice left in the pan after roasting. After removing the meat, carefully pour the juices, including any burnt or crusty pieces, into a heatproof bowl or measuring cup. The caramelized (burnt or crusty) bits are the real source of the roasted flavor. You can get even more by deglazing the pan. To deglaze, pour 1 c water into the pan over medium high heat. Stir constantly, scraping as much as you can from the pan, until the liquid is reduced by at least half.
By this time, the juices you poured out of the pan should have separated with the oil on top and the juices on the bottom. Measure 1/3 c oil into a large saucepan over medium heat. Discard the rest of the fat, saving the juices. (You'll need about 3 c. total juice, including from your deglazing.) Whisk about 3 Tb flour into the fat and cook, stirring constantly for 2-3 minutes, trying to no lumps. Cook another minute or so.
Add the juices slowly, stirring constantly, including in the edges of the pan, until it thickens, usually 5-10 minutes. Salt and pepper to taste.
