Melt the butter and saute the onions and capsicums till softened. Add mushrooms and fry till tender. Stir in the flour and cook for 1 minute. Gradually stir in the milk.Bring to a boil,stirring continuously. Add the stock and simmer for 3 minutes, until thickened. Add the chicken,nutmeg and salt and pepper Simmer for 5 minutes until heated through. Remove from heat and stir in the cream and lemon juice. Serve it with rice, pasta or spoon over mashed potatoes.