Chicken and mushroom hotpot

515g chicken thigh fillets
2-3 shallots,chopped
250g carrots,sliced
1 stick celery,sliced
150g mushrooms,halved
1tbsp chopped fresh thyme
2tbsp gravy granules
1tbsp tomato puree
2tbsp worcester sauce
500g potatoes,peeled
1tsp butter
1tsp oil
1tbsp chopped fresh flat-leaf parsley,to serve

Preheat oven to 190c/170c fan/gas 5.Cut each chicken thigh into 3 pieces.Put in a casserole dish with the shallots,carrots,celery,mushrooms and thyme.Add the gravy to 450ml freshly boiled water and stir to dissolve.Add the puree and worcester sauce and pour into the casserole dish so it reaches the top of the meat and veg.Cut the potatoes into even slices,then arrange overlapping,on top.Cover with lid and cook in oven for 50min.Mix the butter and oil together and brush over the potatoes.Return to the oven and cook uncovered for 20min or until potatoes are golden.Scatter on parsley and serve.