Heat 1 tablespoon of the oil in a large frying pan and cook the chicken and bacon pieces over a high heat for 8–10 minutes or until golden. Meanwhile, in a separate pan, heat the remaining oil and cook the mushrooms over a high heat, stirring, for 5 minutes or until golden and soft. Mix the cooked mushrooms with the bacon and chicken.
In a medium-sized saucepan melt 50 g (2 oz) of the butter, stir in the flour and cook for 1 minute. Remove from the heat and add the milk a little at a time, stirring between each addition until blended. Return to the heat and stir until boiled and thickened. Stir in the mustard and season, then set aside.
Add the parsley and half the thyme to the chicken and bacon. Pour in the sauce, mix and transfer to a shallow gratin dish.
Make up the potato mix according to the instructions on the packet. Add the remaining butter and thyme and beat well to fluff up. Spoon over the chicken and bacon mixture and scatter with the Parmesan. Cook under the preheated grill for 3–4 minutes or until golden and bubbling.