Halve the carrot lengthwise and thinly slice. Coarsely chop the pak choy, keeping the stems and leaves separate. Trim the beans and halve widthwise. Crush or finely chop the garlic. Combine the garlic, teriyaki sauce, sesame oil, half the mirin, 2 tsp white vinegar and ½ tsp sugar in a bowl.
Step 2 — Marinate chicken
DOUBLE THE CHICKEN Put the miso paste, remaining mirin and 2 tsp water in a bowl and whisk until smooth. Put the chicken flat on a board, put your hand on top and halve horizontally into fillets. Add the chicken to the marinade and turn to coat. Bring a medium saucepan of water to the boil for the noodles.
Step 3 — Cook chicken
Heat 1 tbs oil in a large deep frypan over medium heat. Cook the chicken for 2-4 mins each side until golden and cooked through. Remove from the pan and cover to keep warm. Wipe the pan clean.
Step 4 — Stir-fry vegetables
Heat 2 tsp oil in the same pan over high heat. Stir-fry the carrot, pak choy stems and beans for 2-3 mins until slightly softened. Add the sauce and cook, stirring, for 1-2 mins until the vegetables are just tender.
Step 5 — Cook noodles
Meanwhile, cook the noodles in the pan of boiling water for 2 mins or until tender. Drain, then rinse under hot water to prevent the noodles from sticking.
Step 6 — Get ready to serve
Add the noodles and pak choy leaves to the vegetables and cook, stirring, for 1 min or until combined and the leaves are wilted. Slice the chicken. Divide the noodle stir-fry, chicken and any resting juices among bowls. Scatter with a pinch of chilli flakes to serve (chilli is not essential to the dish; you can omit it, if you prefer).