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 quart saucepan, 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, melt 1 tablespoon of the butter over moderate heat. When the foam begins to subside, 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 saucepan 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 saucepan 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 tablespoons of butter.
To serve, ladle the soup into a heated tureen, sprinkle lightly.
with the reserved browned onions and serve the lemon wedges separately.