Wash the tomatoes, cut them in half. Clean the chilli, cut lengthwise, remove seeds, wash and chop. Clean and wash spring onions and cut into rings. Stew everything in hot oil for about 10 minutes.
Grate the cheese roughly. Drain beans and corn, stir into tomatoes and heat. Season to taste with salt, pepper and cumin. Pour into the taco shells and sprinkle with cheese. Add sour cream.