Mix the onions, chillies, garlic, lime juice, coriander and chicken in a shallow non metallic dish. Cover and leave to marinate in the fridge for at least 30 minutes but preferably overnight.
Halve the peppers and grill, cut side down, until the skins are well charred. Leave until cool then peel off the skins, discarding the seeds and stalks. Cut into strips.
Brush a heavy based frying pan with oil and heat until hot. Drain the marinade from the chicken, reserving the onions. Cook the chicken with the onions and peppers over a high heat until browned.
Serve with the warm tortillas, soured cream and salsa.