Green Goddess Sandwich

Name: Green Goddess Sandwich
Ingredients: ½ cup plain whole-milk strained yogurt, such as Greek-style; ½ cup chopped fresh parsley leaves; 2 tablespoons chopped fresh tarragon leaves; 2 tablespoons chopped fresh chives; 1 tablespoon capers, rinsed and chopped; 1 clove garlic, grated; 1 ½ teaspoons grated lemon zest; ¼ cup lemon juice; 1 medium avocado, cut into 8 slices; ¼ teaspoon salt; 4 slices hearty whole-wheat bread (1/2-inch); 1 cup watercress or spinach, divided; 1 cup thinly sliced cucumber, divided; ½ cup alfalfa sprouts, divided