Preheat the oven to 180ºc / Gas 4.
Put the chicken breast at the bottom of a large casserole pot.
Heat the olive oil and butter in a frying pan, add the passata and stir for 1min. Add the garlic and stir for another min.
Add the vinegar and reduce until all has disappeared. Add the wine and simmer until reduced by a third.
Pour in the chicken stock and bring to a simmer, pour over the chicken and cook in the oven for around 40mins or until the chicken has cooked.
Whilst the chicken is cooking, put a cross at the bottom of each tomato and place in a bowl of boiling water for around 5mins. Remove from the water and peel, cut into quarters and remove the seeds, set aside.
Peel some potatoes and boil for around 20mins until soft. Drain, add some butter and milk, mash until no lumps, then beat with a wooden spoon until fluffy. Set aside.
In a small saucepan, gently heat the cream and mustard until just before boiling and set aside.
Remove the pan from the oven, take out the chicken and set aside.
Whisk in the mustard and cream, then simmer to reduce and thicken slightly. Add the diced tomato and tarragon, stir and season to taste.
Serve the chicken with mash and pour over the sauce.
