Preheat the oven to 200C/400F/Gas 6.
Blanch the sweet potato in boiling salted water until nearly cooked, then drain and set aside.
Place the steak into a food processor and blend to a rough mince.
Heat the oil in a frying pan, add the steak mince, mushrooms, and shallot and fry for 2-3 minutes. Add the Worcestershire sauce, red wine, tomato purée and thyme and leave to simmer for a few minutes. Remove the sprig of thyme.
Spoon the mixture into a small gratin dish and layer the sweet potato pieces on top. Sprinkle with the cheese and bake in the oven for 15-20 minutes until golden-brown and bubbling.
Use a mini-blowtorch to crisp up the cheese and serve immediately.
