In a blender, combine chives, mayonnaise, tarragon, yogurt, lemon zest, juice of half a lemon, olive oil, salt, and pepper. Blend until smooth.

In a bowl, squeeze juice from remaining lemon over lettuce and cucumbers. Drizzle with olive oil, season with salt and pepper, and toss gently.

Cut the avocado in half, remove the pit, scoop flesh into a bowl, and mash with a fork, seasoning with salt and pepper.

Spread the smashed avocado on one slice of bread. Layer lettuce and cucumber salad, mozzarella, and sprouts on top.

Spread green goddess dressing on remaining bread slices and place on sandwich, dressing side down. Press gently.

Cut the sandwich in half and serve immediately with vegetable sticks or light soup.