Render the bacon in a large pot over low heat, 5-6 minutes. Remove the bacon and set aside. Pour off the fat leaving approximately 1 tablespoon in the pot.
Add the butter, garlic and leeks. Sweat over medium-high heat until tender and translucent, 5-8 minutes.
Add the stock, potato, celeriac, thyme, salt and pepper. Simmer the soup for about 20-25 minutes or until vegetables are very tender.
Reserve some whole pieces of potato and celeriac – if desired – and purée the soup using a food processor or blender until smooth. Taste and adjust seasonings with additional salt and pepper. If desired, thin the soup with water or stock.
To finish the soup, garnish with reserved bacon, celeriac and potato and drizzle with extra virgin olive oil.