Italian Fish Broth (brodo Di Pesce)

Makes 2 quarts. Peel and coarsely chop the onions. Rinse leeks, remove green tops, and reserve white parts for another use. Heat oil in a 6-quart soup kettle. Add onions and saute over medium-high heat until softened, about 2 minutes. Add fish heads and bones and saute until golden, about 5 minutes. Stir in salt and tomato paste. Add the boiling water to the soup kettle. Cut the celery stalk in half and add it to the soup along with the thyme and leek greens. Simmer for 25 minutes. Strain broth through a fine sieve and return it to the soup kettle; set aside.