Chicken-and-vegetable Soup

• 6 bay leaves
• 4 sprigs thyme (fresh)
• 2 boiling chickens
• 2 onions (yellow)
• 2 leeks
• 2,400 ml water (cold)
• 8 gr salt
• 8 gr butter (unsalted)
• 4 gr black peppercorns (whole)
• 3 gr thyme leaves (fresh)