Combine Cajun seasoning ingredients together in a shallow bowl, and mix well to combine. Reserve 1 teaspoon of seasoning to use in the dressing, and set aside.
Lightly coat each chicken fillet in the seasoning on both sides. Heat 1 tablespoon of oil in a grill pan (or on a grill or skillet) over medium-high heat until hot (not smoking). Sear / grill chicken fillets on each side until golden, crispy and cooked through. (Grill in batches to prevent excess water being released.) Once chicken is cooked, set aside and allow to rest.
Prepare salad with the lettuce leaves, cucumber, tomatoes, avocado slices and onion slices. Slice chicken into strips and arrange on top of the salad.
Mix the reserved cajun seasoning with the mayonnaise and yogurt (or sour cream). At the point, you can add extra mayo or yogurt (sour cream), salt or pepper, adjusting to your tastes.
