To make the BBQ sauce, combine the ingredients in a bowl and set aside.
To make the BBQ chicken, cut the chicken breasts into ½-inch strips and, in a bowl, toss with the barbecue sauce. Cover with cling film and refrigerate for an hour. Preheat a griddle pan or BBQ to a medium heat and grill the chicken until cooked through, 5 to 6 minutes.
Meanwhile, combine the crème fraîche, lemon juice and white wine vinegar to make the salad dressing, season to taste.
Cut the tomatoes in half and place under a medium grill for 15 minutes to dry out. In the meantime, cut the cucumber in half, deseed and then chop on an angle. Cut avocado in half, remove the stone and scoop out the avocado from the peel, chop into thin slices. Remove the outer leaves of the spring onions and also cut on an angle. Cut the lettuce in half and remove the core and then rinse in lightly salted water.
Place the lettuce in a large bowl, add the dressing and toss to coat well then divide into four serving plates, scatter the chicken, tomatoes, cucumber, avocado and spring onions to finish.