Bring a medium saucepan of water to the boil for the noodles. Trim the beans and halve. Thinly slice the capsicum. Halve the carrot lengthwise and thinly slice. Thinly slice the spring onion. Coarsely chop the peanuts. Cut the lime into wedges.
Step 2 — Cook noodles
Cook the noodles in the pan of boiling water for 7 mins or until tender. Drain, then rinse under hot water to prevent the noodles from sticking.
Step 3 — Stir-fry prawns
Meanwhile, heat 2 tsp oil in a medium deep frypan over high heat. Stir-fry the prawns for 2-3 mins until just cooked through. Remove from the pan.
Step 4 — Stir-fry vegetables
Heat 2 tsp oil in the same pan over high heat. Stir-fry the beans for 2 mins. Add the capsicum and carrot and stir-fry for 2-3 mins until tender but still with crunch.
Step 5 — Add egg and sauce
Push the vegetables to one side of the pan, then crack 1 egg into the pan. Cook, stirring, for 30 secs or until the egg is just set. Add the stir-fry sauce, 3 tsp soy sauce and 1 tsp sugar and stir until just combined.
Step 6 — Toss noodles and serve
Add the prawns and noodles to the vegetable mixture and cook, tossing, for 1-2 mins until heated through. Taste, then season with salt and pepper. Divide the noodle mixture among bowls. Scatter over the spring onion, peanuts and ½ tsp chilli flakes* (you can omit the chilli if preferred). Serve with the lime wedges.