Roast Chicken

Ingredients:
One 6-pound roasting chicken, 2 tablespoons unsalted butter, Coarse salt and freshly ground pepper, 2 medium onions, sliced crosswise 1/2-inch thick, 1 lemon, 3 large garlic cloves, crushed plus 2 cloves, thinly sliced, 4 sprigs fresh thyme, 2 sprigs fresh tarragon, 1 cup homemade or canned low-sodium chicken stock