Heat oven to 220C/200C fan/gas 7. Wash the sweet potatoes, prick them with a fork all over, then rub with a little olive oil and season generously. Wrap each potato in tin foil and bake on a baking tray for 45-55 mins until soft.
To make the marinade, put the lime juice, chilli powder, garlic powder, cumin, 1 tbsp olive oil and a generous pinch of seasoning in a bowl. Mix everything together, then pour half the marinade into a second bowl. Add the steak to one of the bowls and mix well. Add the onions and peppers to the other bowl and mix to ensure everything is evenly coated. Cover both bowls in cling film and chill in the fridge until needed.
When the sweet potatoes have about 20 mins left to cook, heat the remaining 1 tbsp olive oil in a large frying pan over a medium heat. Add the onion and pepper mixture, cook for 12-15 mins, stirring often, until they go soft and start to glisten and brown, then tip onto a plate. Wipe the pan clean with some kitchen paper, add the steak and cook for 3-5 mins, stirring often. Add the onions and peppers back to the pan for a few mins before serving, then remove from the heat and stir through most of the coriander.
Mash the avocado in a bowl with a fork. Add a good pinch of salt and the rest of the coriander.
To serve, halve each sweet potato lengthways, mash the insides with a fork, then top with a spoonful of the steak fajita mixture and a dollop of mashed avocado. Enjoy!