Melt the butter in a nonstick frying pan, then add the onion with some salt and pepper and fry for a few minutes on a low heat until they are soft. Add in the harissa and cook for one more minute.
Whisk the eggs in a mixing bowl with more salt and pepper. Add in the coriander and spring onions. Pour into the pan and cook over a low heat until the bottom layer thickens.
Sprinkle over the chilli and the feta, cover and cook for 2-3 minutes, or place under a hot grill until cooked, then serve.