In a soup pot over medium heat, add the olive oil and the onion, celery, carrot, and leek.
Sweat the vegetables until they release a bit of their juice, about 10 minutes.
Add the sage, bay leaves, parsley stems, basil stems, fennel seeds, black peppercorns and salt.
Add 1.5 litres of water.
Turn the heat down to low and bring the water to a simmer and cook for 15 minutes. Remove from the heat and allow the mixture to cool. Keep the mixture in the refrigerator for 24 hours. Strain the mixture through a fine sieve or a sieve lined with a lining of cheesecloth.