Preheat the 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 granules 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 vegetables - add more water if necessary.
Cut the potatoes into even slices, then arrange overlapping, on top. Cover with a lid or foil and cook in the oven for 50 minutes.
Mix the butter and oil together and brush over the potatoes. Return to the oven and cook, uncovered, for 20 minutes or until the edges of the potatoes turn golden. Scatter the parsley on top to serve.