Preheat the oven to 350 degrees
If the turkey has a metal clamp on its legs, remove it. Remove the giblets and neck from the cavity, if included, and reserve for another use or discard. Remove the cavity fat. Then rinse the turkey well and pat dry with paper towels.
In a small bowl, mash the 6 tablespoons (room temp) butter until smooth and season with salt and pepper. Using your fingers, and starting from the cavity end of the turkey, separate the skin from the breast meat, being careful NOT to tear the skin. Gently rub the softened butter evenly over the breast meat, then insert 6 of the sage leaves under the skin, placing 3 leaves on each breast half. Place the skin back into place, then brush the whole exterior of the bird with the melted butter. Season the turkey all over, including the cavity, with salt and pepper. Sprinkle the coffee beans inside the cavity.
To roast the turkey: Brush the bottom of a roasting pan just large enough to accommodate the turkey with the bacon fat, then make a bed of the onion slices in the center of the pan. Place the turkey, breast side up, on top of all the onions.
Place the turkey in the oven and roast for one hour. Baste the turkey with some of the melted butter and add the garlic, the remaining 6 sage leaves and 5 cups of the broth to the pan. Continue to roast the turkey, basting the butter at regular intervals a few times more, for another 1 1/2 to 2 hours. If the turkey is browning too much, tent with aluminum foil. The turkey is finished when a thigh joint is pierced and the juices run clear OR when an instant -read thermometer inserted into the thickest part of a thigh (away from a bone) registers 155 to 165 degrees F.
Remove the turkey from the oven, transfer to a platter and tent with foil. Let it rest for about 20 minutes.
To make the gravy: Before beginning, remove any stray coffee beans that may have escaped from the turkey cavity into the roasting pan. Set the roasting pan with the onion slices and juices on the stove top over medium high heat. (You may need to straddle the pan over two burners). Using a wooden spoon, stir up any browned bits stuck to the pan bottom and continue stirring for a few minutes. Sprinkle the flour evenly over the onion slices and juices and stir until well combined (for about 2-3 minutes).
Add 2 cups of the broth and any juices that have collected around the turkey on the platter, simmer gently, whisking occasionally, until thickened; about 8-10 minutes. If the gravy is too thick, add some more broth. Season with salt and pepper. Pour the gravy into a warmed gravy boat or dish and keep warm. Carve the turkey and serve immediately! Pass the gravy at the table.