Preheat oven to 200°C/400°F/Gas Mark 5. Sauté onions, leek and chillies in sesame oil until onions start to caramelise, stirring well. Add garlic, ginger, star anise, lime leaves and lemongrass (remove the two outer layers and finely slice the thickest end for 4 cm/1 ½ in; discard the rese) and fry a further 1 minute. Add chicken stock and Thai fish sauce, bring to the boil then turn to a gentle simmer.
In a frying pan, brown chicken on all sides, add sweetcorn, toss it a little, then roast them all in oven until just cooked, about 12 minutes. Remove from the oven and slice the chicken when it’s cool enough to handle.
Bring ‘stock’ back to the boil and add beans, boil 1 minute then add sweetcorn and finely sliced chicken. Return to the boil and stir in spinach and lime juice, check seasoning and remove from heat. Divide vermicelli between four warmed bowls and then pour laksa over them. Top with coriander and mint leaves.