Preheat the oven to Gas Mark 5, 190°C. Place the Portobello mushrooms on a baking tray and bake in the preheated oven for 10 minutes. Combine the oil, garlic and parsley and drizzle this over the mushrooms. Return to the oven for a further 10-15 minutes, until the mushrooms are softened.
Split the wholegrain rolls and spread them lightly with mustard. Cover the base of each roll with sliced tomatoes, then top with a cooked mushroom and some salad leaves. Cover with the remaining half of each roll and serve the burgers immediately.
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