Combine the mayonnaise, Greek yogurt/sour cream, lemon juice, honey, salt and pepper in a large mixing bowl. Taste and adjust seasoning. You want the dressing to be tangy, slightly sweet and well-seasoned.
Add the chicken as well as any add-ins to the dressing and mix well.
To assemble the sandwich, slice the bread down the center horizontally then brush with olive oil or melted butter. Toast, cut-side down in a hot pan until golden brown.
Layer lettuce, chicken salad and sliced avocado onto the bottom piece of bread and season with salt and pepper. Top with the remaining piece of bread then slice and serve.