Method
In a large pot, heat the chicken stock over a low flame and let it simmer. In another pan, heat the butter and olive oil together. Now add the finely chopped onions, garlic, and celery and fry them very slowly at low heat. Stir continuously with a wooden spoon until the vegetables have softened and the onions have turned translucent. Take care not to overcook the onions and turn them golden brown.