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Tip the noodles into a bowl and cover with boiling water. Leave to stand for 10 mins, then drain, rinse in cold water and set aside.
In a jug, mix the Marmite with 500ml boiling water. Set aside while you cook the vegetables.
Heat the oil in a saucepan, then add the mushrooms and leek. Cook for 10-15 mins until softened and beginning to colour, then add the soy sauce and Marmite and water mixture and stir. Bring to the boil for 5 mins.
Divide the noodles between two deep bowls, then ladle over the hot broth. Top with the chilli slices, mint leaves and peanuts, and serve with some sriracha on the side.