Place the chopped onion in a large sauté pan with the butter and cook gently for 10 minutes without colouring. Add the chicken breasts, sprinkle over the roughly chopped tarragon, orange zest and juice and then the wine. Bring up to the boil and then reduce to a simmer. Put on the lid and cook gently for 25 minutes or until the chicken is cooked through and firm.
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.