In a blender, combine chives, mayonnaise, tarragon, yogurt, the zest and juice of half a lemon, and 2 tablespoons of olive oil. Season with salt and pepper, then blend until smooth.

In a medium bowl, toss chopped lettuce and sliced cucumbers with the juice from the other half of the lemon, remaining olive oil, salt, and pepper until well-coated.

On two slices of bread, mash the avocado with a fork and season with salt and pepper.

Layer seasoned lettuce and cucumber on top of the mashed avocado, followed by slices of mozzarella and a handful of sprouts.

Spread the remaining slices of bread with about 1/4 cup of the dressing and press down on the layered sandwich components to complete.