Heat 2 tablespoons butter in a large pot over medium. Add onion and carrots; cook, stirring occasionally, until starting to soften, about 5 minutes. Add broth, tomatoes, lentils, salt, and sumac, if using. Bring to a simmer over high. Reduce heat to medium-low; cover and simmer until lentils have softened, about 15 minutes.
Remove from heat and stir in lemon juice and remaining 2 tablespoons butter. Divide soup among 4 bowls and top with yogurt. Sprinkle with lemon zest, dill, and more sumac, if using.
