Heat a griddle pan (or ordinary frying pan). Brush the chicken breasts with sunflower oil, place them in the pan and cook for 8–10 minutes on each side. Remove from the pan and slice into strips.
To make the salsa, place the avocado in a bowl and add the onion, chillies, garlic and coriander. Mix together, adding the lime juice, olive oil and seasoning.
Pile some of the salsa in each of the taco shells and top with a few strips of chicken. Serve the remaining salsa separately, with a bowl of soured cream to spoon on top of the tacos just before eating.