Wash the potatoes, but leave the skins on. Put into a large saucepan, with a little salt and water. Cook until quite tender. About 5 minutes before the end of cooking the potatoes, pop in the peas. Drain both.
Heat the milk and cream up. Lightly mash the potatoes and peas, season ad then add the butter. Stir in enough of the cream and milk to soak in, but not too much as to make it sloppy.
Remove the chicken and keep warm. Bring the heat up and boil the sauce. Add the creme fraiche and season. Pour over your chicken breasts and sprinkle over a little more tarragon, and orange zest. Stir the mint into the potato and serve with the chicken breasts.