Heat the oil in a saucepan or stockpot. Add the onion and thyme; sauté until the onion is soft, but not brown. Add the tomatoes and stir briefly.

Add the flour, and stir for about 3 minutes, or until the flour is fully incorporated into the mixture.

Stir in the sugar, chicken stock, and tomato paste. Simmer, partially covered, for 30 minutes, stirring occasionally to prevent sticking.

Puree the soup mixture in a food processor and return ro the pot. Season with salt and pepper, to taste.

Just before serving, add the basil. Serve with a dollop of yogurt, if desired.