On a cutting board, dice the onion and tomatoes with a serrated knife.

Heat butter in a medium-sized pan and sauté the onion until transparent, about 5 minutes. Add tomatoes, chicken broth, 1/3 cup of the dill, sugar, salt and pepper and bring to a boil. Lower the beat and simmer for 20 minutes.

Serve hot. Ladle into soup bowls and top with a spoonful of yogurt. Sprinkle with remaining fresh dill.