1 teaspoon olive oil
4 medium leeks (white and pale green parts, about 2 cups), chopped
1 small onion (1/4 pound), thinly sliced
2 large shallots (1/4 pound), thinly sliced
salt and pepper to taste
1 1/2 cups water
1 large potato (6 ounces) such as Yukon Gold, peeled and cut into 1/2-inch cubes
1 cup chicken broth
1/2 cup grated Gruyère (2 ounces)
2 teaspoons balsamic vinegar
Heat oil over moderate heat until hot but not smoking; add leeks, onion, and shallots and season to taste. Cook about 15 minutes, stirring frequently, until edges are golden brown. Add 1/2 cup water and deglaze skillet, scraping up brown bits. Add potato, broth, and remaining cup water to onions. Simmer, covered, stirring occasionally, until potatoes are very tender.
Pour about 1 cup of soup into blender, puree, and return to pot. Adjust seasonings and serve sprinkled with cheese and drizzled with vinegar.
Makes about 4 cups.
Adapted from Gourmet (via the Epicurious website).