Wash the lentils in a large sieve or colander set under cold running water until the draining water runs clear.
In a heavy 4 to 5 qt pot, bring the stock to a boil over high heat. Add the lentils, onion, tomato and garlic, reduce the heat to low, and simmer partially covered for 45 minutes, or until the lentils are tender.
Meanwhile, in a small skillet, heat 1 tb of the oil over moderate heat. Add the chopped onions and, stirring frequently, cook for 10 minutes, or until they are soft and deeply browned. Set aside off the heat.
Puree the soup through a food mill or pour the entire contents of the pot into a sieve set over a deep bowl and force the ingredients through with the back of a large spoon, pressing down hard on the vegetables before discarding the pulp. Return the soup to the pot and, stirring constantly, cook over low heat for 3 or 4 minutes to heat through. Stir in the cumin, salt and pepper, and taste for seasoning. Just before serving, stir in the remaining 3 tbs of oil.
To serve, ladle the soup into a heated tureen, sprinkle lightly with the reserved browned onions and serve the lemon wedges separately.