Place tomatoes, tomatillos, garlic and chillies in a saucepan and add enough water to cover. Bring to the boil and cook for 15 minutes or until vegetables are tender. Remove from heat, drain and allow to cool.
Place avocados, the cooked vegetables, sour cream and coriander in a liquidiser or food processor then blend until smooth. Season to taste with salt. Cover and refrigerate until ready to serve.