Melt butter over medium-high heat in a large stockpot or Dutch oven. Saute onions with salt until deep brown, about 10 minutes. Reduce heat to low and add garlic, ginger, and pepper. Cook until aromas are released, stirring constantly, about 2 minutes. Add lentils and cook an additional 3 minutes, constantly stirring so the beans are evenly cooked.
Add chicken stock, bring to a boil, reduce to a simmer, and cook, uncovered, about 1 hour 15 minutes. Stir soup regularly to ensure even cooking so the beans do not burn. When done, beans should be soft inside, with no chalkiness. Serve immediately, garnished with a dollop of yogurt.
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