Cut chicken breast into bite-sized pieces. Wash, core, and cut red and yellow peppers into thin strips. Peel and grate carrots. Clean leek and cut into thin rings.
Heat sesame oil in a wok over medium-high heat. Fry chicken pieces for 8-10 minutes until cooked through (internal temperature 74°C/165°F). Remove and set aside.
Fry the vegetables in the wok for 4-5 minutes. Return the chicken to the wok.
Add soy sauce and pour in the coconut milk. Stir in the curry paste and add fish sauce to taste if desired. Heat through for 2 minutes, stirring gently.