Heat the butter in a large saucepan and add the potatoes. Stir to coat in the butter then add the boiling water and a generous pinch of salt. Cook over a low heat for about 5 minutes then remove from the heat and set aside.
Heat the oil in a frying pan, add the onion and cook for about 7 minutes until soft and starting to go golden . Add the carrots and cook for a further 3 minutes until they start to colour.
Add the peppers and saute for a few minutes then add the tomatoes. Keep stirring from time to time so the mixture does not burn.
Add the sauted vegetables and the rice ( if using) to the potatoes and stock. Cook for about 10 minutes until the rice and potatoes are tender.
Lower the heat so that the suop is barely bubbling, add the chilli flakes and then gradually add the eggs ( if using), whisking them in thoroughly.
Finally whisk in the garlic and add the tarragon. Serve immediately.