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Put the celeriac in a bowl, add a splash of water and cover with cling film. Pierce the cling film and microwave on High for 10 mins or until really tender.
Meanwhile, put the chicken breasts between 2 sheets of cling film and lightly bash with rolling pin until they are an even thickness. Dust with the mustard powder. Heat the oil in a large frying pan, add the chicken breasts and brown on both sides. Add the garlic and sage to the pan, stirring in the gaps between the chicken, for 30 secs- 1 min, then crumble in the stock cube. Pour in 100ml hot water and add 100g crème fraîche. Stir the sauce around the chicken, then cover the pan with a lid and cook over a medium heat for 8 mins or until the chicken is cooked through. Stir through the mustard and season with plenty of black pepper.
While the chicken cooks, drain any liquid from the celeriac, season, add the remaining 2 tbsp crème fraîche and mash (or blitz to a purée in a food processor). Serve with the chicken and some green veg.