1.Preheat the oven to gas 6, 200°C, fan 180°C. Boil the potatoes for 15 mins or until very tender. Drain and set aside in the colander to dry a little.
2.Meanwhile, heat 1 tbsp oil in a frying pan, add the shallots and cook over a low heat for 7-8 mins, adding the garlic and thyme for the final min. Tip into a bowl and mix in the breadcrumbs, pine nuts and half the cheeses; season.
3.Put the mushrooms and tomatoes on a baking tray, drizzle with the remaining oil and the balsamic glaze. Top the mushrooms with the breadcrumb stuffing and bake for 15 mins. Remove the tomatoes; keep warm. Preheat the grill to high. Divide the remaining cheese between the mushrooms and grill until melted.
4.Meanwhile, melt the butter in a frying pan and sauté the leeks with a pinch of salt until softened. Add the crème fraîche and the butter from the leeks to the potatoes; mash until smooth. Stir through the leeks. Serve the mash with the mushrooms and tomatoes.