You'll be tempted to eat up the garlicky chard and onion sauté before it goes into the soup. And that’s just fine because it tastes so delicious like that. But if it’s soup you’re after, just taste and then stir the fragrant mix into the lentils. This soup tastes best when it’s not piping hot, but has cooled down a bit and flavors can really shine through. A classic rushta also contains cooked pasta noodles like linguine, which can easily be added to the recipe below. Once the chard mixture is added to the lentils, the longer it is cooked the more the chard will lose its vibrant green color and the more the lentils will soak up the liquid, so keep that cooking to a minimum.
In a 2- or 3-quart soup pot, bring lentils, 4 cups water, and ½ teaspoon of salt to boil over high heat. Reduce heat and simmer, covered, just until lentils are tender, about 15 minutes.
Meanwhile, in a medium sauté pan, heat the olive oil over medium heat. Add the onions and season with 1/2 teaspoon of salt, the coriander, and pepper. Sauté until very soft and translucent. Add the minced garlic and sauté just until fragrant, about 2 minutes. Add the Swiss chard and cilantro and sauté until bright green and tender. Sprinkle with flour and stir to combine, still over the heat.
Add the Swiss chard mixture to the lentils, stir in the lemon juice, and heat through for just a few minutes. Taste and adjust seasoning.
Serve warm, but not piping hot, for best flavor. Top bowls of soup with chopped cilantro.