Place chives, mayonnaise, tarragon, and yogurt in a blender - add zest and juice of 1 lemon half, then add 2 Tbsp oil; season with salt and pepper. Purée until smooth.
In a medium bowl, squeeze juice from the remaining lemon half over lettuce and cucumbers. Drizzle with oil; season with salt and pepper. Toss to coat.
Divide and smash avocado between 2 slices of bread; season with salt and pepper. Arrange lettuce and cucumbers over. Top with mozzarella and sprouts. Spread the remaining two slices of bread with ¼ cup of green goddess dressing and close sandwiches.
recipe: bon appétit