In a bowl, combine couscous, bell peppers, onion, lemon juice, 2 tablespoons olive oil and 1/3 cup water. Season with salt.
Refrigerate until couscous is tender, about 2 hours.
In a large skillet over high heat, heat canola oil. Cook chicken until golden brown on the bottom, 3 to 4 minutes. Reduce heat to medium-low. Flip chicken and cook through, 6 to 8 minutes.
Cool chicken; cut into 1/2-inch dice.
In a blender set to medium, blend lime juice, vinegar, mustard, honey and soy sauce. Slowly add remaining 2 tablespoons olive oil. Season with salt.
Break up lettuce heads and combine leaves with arugula in a bowl. Divide lettuce, couscous, chicken and mango among 4 plates. Dress salad with mustard vinaigrette.