Peel the mushrooms, leaving the stalks intact, then place in an ovenproof dish. Heat the oil in a frying pan, add the onions and fry for 5 minutes until softened. Stir in the sugar and cook over a medium heat for 5–10 minutes until very soft and lightly caramelized.
Stir in the thyme leaves, balsamic vinegar, fresh and sun-dried tomatoes and some seasoning. Divide the mixture between the mushrooms then cover and chill until required.
Put two-thirds of the cheese in a small saucepan, add the crème fraîche and a little seasoning then cover and chill until required.
When ready to serve, remove the cover and bake the mushrooms in a preheated oven, 200°C (400°F), Gas Mark 6, for 15 minutes until piping hot. Warm the cheese and crème fraîche mixture, stirring until the cheese has melted. Add the salad leaves to the sauce and cook for 30 seconds.
Toast the bread and divide between 4 serving plates. Scoop the wilted leaves out of the sauce and on to the toast, then put the mushrooms on top of the leaves. Sprinkle with the remaining gorgonzola then drizzle the sauce around the edges of the plates.