Preheat oven to 400°F Bake the potatoes for 1 hour, or until tender.

Meanwhile, make the filling. Heat a large saucepan and add the beef a handful at a time, so that it sears and browns.

Add the onion and garlic, cook for 1 minute, then add the mushrooms, stock and tomato paste, stirring well. Heat and simmer, partially covered, for 20-25 minutes.

To serve, split the baked potatoes and share the filling between them. Top with watercress, sliced red onion and cherry tomatoes, then sprinkle the feta cheese over the top.