Step 1: In a small bowl, whisk together mayonnaise, sour cream (or Greek yogurt), chopped parsley, chives, tarragon (if using), minced garlic, lemon juice, and Dijon mustard. Season with salt and black pepper to taste. Taste and adjust seasonings as needed to achieve your desired flavor.
Step 2: In a medium bowl, combine the finely diced cucumber, celery, green bell pepper, radishes (if using), and shredded romaine lettuce or spinach.
Step 3: Pour about half of the prepared Green Goddess dressing over the chopped vegetables. Stir gently to combine, adding more dressing a tablespoon at a time until the vegetables are well coated but not soggy. You may not need all the dressing.
Step 4: Cover the bowl and refrigerate the Green Goddess salad for at least 15 minutes to allow the flavors to meld and the salad to firm up slightly. This makes it easier to build the sandwich.
Step 5: Lightly toast your bread slices if desired. Spoon a generous amount of the chilled Green Goddess salad onto one slice of bread. If desired, you can add an extra layer of fresh lettuce or spinach before topping with the second slice of bread.
Step 6: Carefully slice the sandwich in half and serve immediately. Enjoy your fresh and flavorful Green Goddess Salad Sandwich!