Bring a large pan of water to the boil, add the rice and cook for 12-13 minutes, adding the runner beans and peas for the final 3 minutes of cooking time. Drain and rinse under cold water, then transfer to a large bowl.
Meanwhile, heat a griddle pan over a high heat. Rub the chicken fillets and pepper slices with 1 tbsp oil. Cook on the griddle for 10 minutes, turning once, until the chicken is cooked through with no pink remaining and the peppers are charred and tender. Slice the chicken breasts.
Whisk together the remaining olive oil, the mustard, 50ml water and the lemon juice, then season.
Add the griddled pepper to the rice and runner bean mixture and stir through the spinach. Pour over the dressing and toss to combine. Divide the rice salad between plates, top with the sliced chicken and garnish with the lemon zest.