Remove peel from celery root and dice. Chop carrots and leek. Crush garlic cloves and quarter onions. Add vegetables to a large pot filled with cold water. Add thyme rosemary, allspice, bay leaves, black peppercorns, salt, and sugar to pot.
Cover pot with lid and bring to a boil, then turn down heat to low and leave to simmer for approx. 1 hr.. Pass stock through a fine sieve over a large bowl. Store in an airtight container in the fridge for up to 1 week.