Heat the butter in a saucepan and sauté the garlic, onion and ginger until the onion is softened, about 3 minutes. Add the tomatoes and simmer until the juices reduce slightly, about 7 to 10 minutes.
Add the stock and continue cooking for another 5 minutes or so, then purée (and sieve if desired).
Stir the yogurt and add several spoonfuls of the purée