Place lentils in a fine-mesh sieve and rinse until cold running water. Set aside.
In a stock pot, combine the oil, bouquet garni, onion (or leek) swiss chard stems, and 1 teaspoon salt. Sweat-cook, covered, over low heat until soft but not browned- for about 3 minutes.
Add the stock and bring to a simmer over moderate heat. Add the lentils, curry powder, and cayenne and stir. Simmer, covered, until the lentils are tender, about 20 minutes. (Cooking time will depend upon the freshness of the lentils–older lentils take longer to cook).
Add the chard leaves and the chickpeas and cook until the leaves are wilted, about 5 minutes more. Remove the bouquet garni. Taste for seasoning. (this means add S & P to taste).
While the stew is cooking, toast the cumin. Watch carefully! They can burn quickly. Place the cumin seeds in a small, dry skillet over moderate heat. Shake the pan regularly until the cumin seeds are fragrant and evenly toasted, about 2 minutes. Transfer the cumin to a large plate to cool. Set aside.
Mix together yogurt and cumin.
Divide soup among the warmed soup bowls. At the table, garnish with a spoonful of yogurt and a sprinkling of toasted cumin.