Rinse the chicken, pat dry and cut into thin slices. Rinse and halve bell pepper and chile pepper, remove seeds and ribs, and cut both into thin strips. Peel and finely chop onion and garlic. Rinse, core and cut tomatoes into cubes. In a pan, fry chicken in hot oil. Remove from pan. Add pepper, chile, onion and garlic and sauté 1-2 minutes. Add tomatoes and simmer until slightly thickened, about 5 minutes. Add chicken and simmer over low heat, 2-3 minutes. Season with salt, coriander and pepper.
To make guacamole: Halve avocados, remove pits and scoop out flesh. Mash finely with a fork. Season with lime juice, salt and pepper.
Rinse lettuce and tear into small pieces. Rinse tarragon, shake dry, and strip leaves from stems. Divide chicken filling among taco shells and top with lettuce, herbs, guacamole, sour cream. Season with pepper and serve.