Notes: If making up to 1 day ahead, cover leeks and sauce, chill, then bring to room temperature or reheat to serve.
Trim off and discard leek roots and tops, making each leek 6 inches long. Rinse well.
In a 10- to 12-inch nonstick frying pan, combine onion and garlic. Stir over medium-high heat until onion is limp, 4 to 5 minutes.
Add wine, broth, lemon slices, tomatoes, and parsley to pan. Lay leeks in pan. Cover and bring to boiling over high heat. Turn heat to low and simmer until leeks are tender when pierced, about 10 minutes.
With a slotted spoon, transfer leeks to a shallow rimmed dish.
Discard lemon slices and parsley. Boil tomato mixture over high heat until reduced to about 1 cup, 3 to 5 minutes.
Pour sauce across center of leeks. Garnish with lemon peel and season to taste with salt and pepper. Serve warm or at room temperature.