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 tablespoon of the oil and 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. You may need to do this in 2 batches. Slice the chicken breasts.
Make the dressing: whisk together the remaining olive oil, the mustard, 50ml of water and the lemon juice, then season with freshly ground black pepper.
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. Serve with the lemon wedges.