In a small bowl, combine the Greek yogurt, lemon juice, chives, parsley, dill, garlic powder, salt, and pepper. Mix until well combined to create the Green Goddess dressing.
Spread the mashed avocado evenly on one slice of the whole grain bread.
Spread the Green Goddess dressing on the other slice of bread.
Layer the cucumber slices, alfalfa sprouts, shredded carrots, and radishes on top of the avocado.
Place the slice with the dressing on top of the veggies to form a sandwich.
Cut the sandwich in half and serve immediately, or wrap it in parchment paper for a portable lunch.