1. Melt butter in large saucepan over medium-high heat. Add onion; saute 2 minutes. Mix in carrots. Add broth; bring to boil. Reduce heat, cover, and simmer until carrots are very tender, about 20 minutes.
2. Stir cumin seeds in small skillet over medium-high heat until fragrant, 4 to 5 minutes; cool. Finely grind in spice mill.
3. Remove soup from heat. Puree in batches in blender until smooth. Return to same pan.
4. Whisk in honey, lemon juice, and allspice. Season with salt and pepper. Ladle soup into bowls. Drizzle yogurt over; sprinkle generously with cumin.