Cream of Lentil Soup (Pure de Lentejas)

1 pound dried quick-cooking lentils, rinsed in cold water and picked over
2 quarts water
1 ham bone
1 medium-size onion stuck with 3 cloves
1 rib celery, without leaves, chopped
2 medium-size ripe tomatoes, quartered
1 clove garlic
1 large leek, the white bulb and tender part of the green stem, cleaned well and sliced
1 medium-size carrot, scrubbed and diced
1 medium-size all-purpose potato, peeled and diced
1 bay leaf
1 teaspoon ground cumin
1 tablespoon red wine vinegar
salt and freshly ground black pepper to taste
2 cups milk

In a large saucepan, over low heat, combine all the ingredients except the salt, pepper, and milk, and simmer, covered, 45 minutes to 1 hour, adding more water if necessary.

Remove the ham bone, onion, and bay leaf, and discard. Pour the soup, in batches, into a food processor fitted with a steel blade, and process until smooth. Return the soup to the saucepan and add the salt and pepper.

Over low heat, gradually add the milk, stirring to blend. Cook 10-15 minutes, correct the seasonings, and serve. The soup thickens upon standing, so to reheat, thin it out with a little stock or milk.