Divide the turkey into parts: Reserve giblets and neck for gravy. Remove hindquarters from turkey; separate drumsticks from thighs. Leaving the skin on the thighs, remove bones; reserve bones for gravy. Butterfly and lightly pound thigh meat until 1⁄2" thick; chill until ready to use. Remove wings and, using kitchen shears, remove backbone from turkey so only bone-in breast remains; reserve wings and backbone for gravy.
For the brined turkey: Boil salt, honey, peppercorns, lemons, bay leaves, garlic heads, parsley, thyme, and 2 gallons water in a large stockpot. Let brine cool. Add drumsticks and breast; cover and chill at least 3 hours or overnight.
Roast the brined turkey: Heat oven to 425°. Remove drumsticks and breast from brine and pat completely dry using paper towels; transfer to a roasting pan fitted with a rack. Brush with clarified butter; season with salt and pepper. Add reserved giblets, neck, thighbones, wings, and backbone, along with the 2 garlic cloves, carrots, leek, and onion to bottom of pan; roast until an instant-read thermometer inserted into the breast reads 165°, about 2 hours. Let drumsticks and breast rest 15 minutes before carving breast; keep warm. Discard grease from pan. Return pan to oven and roast until giblets, bones, and vegetables are browned, 45–50 minutes; set pan aside.
Make the mousse for the roulades: Purée ground turkey in a food processor. With the motor running, slowly add egg whites, then the cream, crème fraîche, rosemary, thyme, salt, and pepper until mixture is smooth. Transfer mousse to a bowl and cover with plastic wrap; chill 30 minutes.
Assemble and cook the roulades: Heat a large saucepan of water until a deep-fry thermometer reads 180°. Lay thigh meat flat on a work surface and season with salt and pepper. Divide mousse among thighs and spread widthwise down center of each thigh. Working from one short side, roll each thigh, encasing the mousse, into a tight package; tie with butchers' string. Wrap roulades separately using a triple layer of plastic wrap until airtight; twist ends and tie with string. Gently lower roulades into water; poach until filling is firm, 35–45 minutes. Discard plastic wrap; pat roulades dry using paper towels. Reduce oven to 350°. Melt butter in an ovenproof 12" skillet over medium. Cook roulades, turning as needed, until browned, 18–20 minutes. Transfer skillet to oven; cook roulades, basting often with pan juices, until an instant-read thermometer inserted into the thigh meat reads 165°, 20–25 minutes. Let rest 15 minutes before carving.
For the gravy: Place pan with reserved giblets, bones, and vegetables over medium-high. Add stock and cook, scraping up browned bits from bottom of pan, 2–3 minutes; strain stock. Cook sausage in a 6-qt. saucepan over medium-high until browned, 10–12 minutes. Sprinkle in flour; cook 2 minutes. Stir in stock; boil. Reduce heat to medium; cook until gravy is slightly thickened, 5–7 minutes. Arrange sliced turkey breast and roulades on a serving platter with drumsticks. Garnish with chives, if you like; serve gravy on the side.