In a small bowl stir together thyme, marjoram, savory, lavender, and celery salt; remove 1 tablespoon of the mixture and set aside. Loosen skin from breast; rub the remaining herb mixture over meat under breast skin and inside cavity of capon.
Twist wing tips under back. Tie drumsticks to tail. Place capon, breast side up, on a rack in a shallow roasting pan. Brush bird with oil. Insert a meat thermometer into the center of one of the inside thigh muscles. (The thermometer should not touch the bone.) Roast in a 350 degree F for 2 to 2-1/2 hours or until bird is nearly done. Sprinkle the reserved herb mixture over entire bird. Return to oven and roast, uncovered, about 15 minutes more or until thermometer registers 180 degree F.
Meanwhile, 20 minutes before bird is done, in a very large skillet, melt the butter over medium heat. Add red onions, pearl onions and brown sugar. Cook, covered for 20 to 30 minutes or until onions are very tender, stirring occasionally.
Transfer capon to a serving platter, reserving juices in pan. Cover and let stand for 15 minutes before carving. Pour pan juices into a glass measuring cup. Skim off fat. Measure 1/4 cup juices; discard remaining juices. Wipe out roasting pan with paper towels. Place onion mixture in same roasting pan; add juices and balsamic vinegar and toss to coat. Roast, uncovered, for 15 minutes. Serve capon with onions. Makes 8 to 10 servings.