1. Melt unsalted butter in a large saucepan over medium-high heat. Add scallions, garlic, and curry powder. Cook until vegetables are golden and soft, about 4 minutes. Stir in tomato paste and lentils and cook until slightly caramelized, 1 to 2 minutes. Add tomatoes and 1 3/4 teaspoons salt. Add enough water to cover by 1/2 inch; bring to a boil over high heat. Reduce heat to medium-low and simmer until lentils are tender, 25 to 40 minutes. If lentils begin to look dry while cooking, add more water as needed.

2. Heat a small skillet over medium heat. Add coconut flakes and mustard seeds; season with salt and cook until coconut is golden, about 3 minutes.

3. Divide lentils evenly among six serving dishes. Top each with 2 teaspoons salted butter, yogurt (if using), cilantro, and coconut mixture. Serve immediately.