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