Preheat the oven to 200°C/400°F/gas mark 6.
Put the sweet potatoes on a large sheet pan with sides. Drizzle over ½ teaspoon of the olive oil. Bake in the preheated oven for 15 minutes.
Meanwhile, mix the peppers, onions, remaining 2½ teaspoons olive oil and the fajita seasoning together in a bowl.
Once the sweet potatoes have been baking for 15 minutes, add the pepper and onion mix to the sheet pan and stir. Bake for another 15 minutes, then add the chickpeas for the last minute and stir well.
To make the avocado mayonnaise, put all the ingredients except the olive oil into a food processor and process to a paste. Add the olive oil and process again. Adjust the lemon juice, vinegar and salt and pepper to your preferred taste.
Serve the fajitas with avocado mayonnaise or coconut yogurt and either wraps or rice.