Meanwhile, heat the vegetable oil in a small skillet. Add the onion and garlic and cook over moderate heat, stirring occasionally, until the onion softens, about 10 minutes. Transfer the cooked onion and garlic to a blender. Add one-third of the ancho chiles and the tomatoes with their juices and blend for 1 minute. Strain the mixture into a medium saucepan. Add the stock, oregano, thyme and marjoram and bring to a boil, then simmer for 10 minutes. Season the soup with salt.