Chicken Fajitas

For the vegetables:
2 medium onions, peeled and cut into thin wedges
2 red peppers, de-seeded and sliced
2 tbsp olive oil
For the chicken:
4 chicken breasts
30g (1oz) sachet fajita seasoning
For the guacamole:
2 medium avocados, de-stoned and peeled
Juice 1 lime
1-2 green chillies, de-seeded and finely chopped
2 tbsp freshly chopped coriander
Salt and freshly ground black pepper.