Heat oven to 200C/180C fan/gas 6. In a bowl, whisk the miso paste, mirin, soy sauce, garlic and half the sesame oil until smooth and combined. Line a baking tray with a large sheet of foil, place the mushrooms in the middle and scrunch up the sides to almost encase the mushrooms. Pour over the miso mix, turning the mushrooms to ensure they are well coated. Scrunch the packet of foil closed and bake for 30 mins.
About 5 mins before the mushrooms are ready, heat the remaining oil in a wok, stir-fry the noodles until hot through then divide between 4 bowls. Top each bowl with 1 or 2 mushrooms and pour over the sauce from the foil. Scatter with sesame seeds and spring onions.