Preheat the oven to 375°F (190°C). Pull out the mushroom stalks and discard, then wipe the tops with paper towel (don't wash or peel the mushrooms). Rub the tops of the mushrooms lightly with a little olive oil, then place them upside down in a non-stick, shallow roasting dish. Season with freshly ground black pepper and bake for 10 minutes. Remove from the oven.
Slice the cheese thinly and place in the mushroom cups. Mix together the breadcrumbs and nuts and sprinkle the mixture over the cheese. Return the mushrooms to the oven and bake for a further 5 minutes until the cheese just melts.
Meanwhile toast the bread. There's no need to butter the toast, but you can brush it with a little olive oil, if you like.
Trim the ends from the tomato, then cut it into four even slices. Lay a slice of tomato on each slice of toast, then top with a hot baked mushroom, cheese-side up, and serve at once.
Nutrients per serving: 404 calories, 18 g protein, 23 g fat (8 g saturated fat), 28 g carbohydrates (7 g fibre)
Best Health tip: Choose either large, white, flat mushrooms or the firmer textured brown flat type, called portobello, which has a stronger, nuttier flavour.