Perfect Roast Chicken

Ingredients:
1 six-pound roasting chicken, 2 tablespoons unsalted butter, Salt and freshly ground black pepper, 2 medium onions, peeled and sliced crosswise 1/2 inch thick, 2 lemons, 3 large cloves garlic, peeled, 4 sprigs fresh thyme, 1 cup Homemade Chicken Stock, or canned low-sodium chicken broth, skimmed of fat