Stuffed Turkey with Lemon, Oregano, and Red Onions

Preheat oven to 425°F with rack in lowest position.

Stir together all butter ingredients with 1 teaspoon salt and 3/4 teaspoon pepper.

Rinse turkey inside and out, then pat dry. Put turkey on rack in roasting pan.

Sprinkle turkey cavities with 1/2 teaspoon salt (total).

Starting at large cavity, gently slide your fingers between skin and flesh of breast and legs to loosen skin, being careful not to tear skin. Using a tablespoon measure, slide 2 tablespoons butter under skin of each breast half and 1 tablespoon on each drumstick (6 tablespoons total), pushing butter out of spoon with a finger on outside of skin.

Melt remaining lemon butter in a small saucepan over medium heat. Toss onion wedges with 2 tablespoons melted lemon butter.

Spoon stuffing into large cavity of turkey. Fold neck skin under body, then tie drumsticks together with kitchen string and tuck wings under body.

Brush turkey all over with half of remaining melted lemon butter and sprinkle with 1 1/2 teaspoons salt and 1 teaspoon pepper. Roast until skin is golden brown, about 30 minutes.

Reduce oven temperature to 350°F. Rotate pan 180 degrees.

Brush remaining melted lemon butter over turkey and roast 30 minutes more. Baste turkey and scatter onion wedges around it, then roast, basting turkey and rotating turkey 180 degrees every 30 minutes (if pan becomes completely dry, add 1/2 cup water; if skin becomes too dark, loosely cover turkey with foil) until an instant-read thermometer inserted into fleshy part of thighs (test both; close to but not touching bone) registers 170°F and stuffing registers at least 165°F, about 2 to 2 1/2 hours more (total roasting time: 3 to 3 1/2 hours).

Transfer turkey and onions to a platter, reserving juices in pan.

Let turkey stand, uncovered, 30 minutes (temperature of thigh meat will rise to 175°F).

Straddle roasting pan across 2 burners, then add 1 cup turkey stock and deglaze pan by boiling over high heat, stirring and scraping up brown bits, 2 minutes. Strain through a fine-mesh sieve into 2-quart measuring cup and skim off and reserve fat. (If using a fat separator, strain pan juices through sieve into separator and let stand until fat rises to top, 1 to 2 minutes. Carefully pour pan juices from separator into measuring cup, reserving fat.)

Stir together flour and 6 tablespoons reserved fat (if you don't have enough fat, add butter) in a heavy medium saucepan, then cook roux over medium heat, whisking, until pale golden, about 3 minutes.

Add pan juices and remaining 3 cups stock in a fast stream, whisking constantly to prevent lumps, then bring to a boil, whisking. Stir in any turkey juices from platter and briskly simmer, whisking occasionally, until gravy is thickened to desired consistency, 10 to 12 minutes. Stir in lemon juice and salt and pepper to taste.

Lemon butter can be made 3 days ahead and chilled, covered. Bring to room temperature before using.