Melt the butter in a shallow, flameproof casserole. Add the onion and chicken and fry gently for 6–8 minutes, stirring frequently, until lightly browned.
Add the mushrooms and fry for a further 5 minutes. Sprinkle in the flour, then add the stock, mustard, parsley and a little seasoning. Bring to the boil, then reduce the heat and stir in the cream and beans.
Slice the polenta very thinly and arrange, overlapping the slices, on top of the chicken. Sprinkle with the cheese and a little black pepper.
Place in a preheated oven, 190°C (375°F), Gas Mark 5, for 30–40 minutes until the cheese is melted and beginning to brown. Serve with a leafy salad.