Place a large soup pot over medium-high heat with the EVOO. Once the oil is hot, add the onions and cook until they begin to caramelize. Add the cauliflower, celery, bay leaf and thyme springs, and cook, stirring frequently for 8-10 minutes, until the veggies are nice and brown.
Add the garlic, some salt and freshly ground black pepper, and continue to cook for 2-3 minutes. Add about 1 cup of the chicken stock and scrape up any brown bits on the bottom of the pot.
Remove the pot from the heat and discard the thyme stem and bay leaf. Working in batches, transfer the veggies to a food processor. Puree until completely smooth, adding a little splash of stock to moisten mixture if necessary.