For the sauce:

• ¼ cup Greek yogurt
• ¼ cup mayonnaise
• 1 clove garlic, grated or pressed
• 3 tablespoons fresh chives, finely chopped
• 3 tablespoons tarragon, finely chopped
• Zest and juice of ½ lemon
• 1 tablespoon olive oil
• ¼ teaspoon salt
• ¼ teaspoon ground black pepper

For the sandwiches:

• 4 thick slices whole-wheat bread (½ inch)
• 1 cup arugula (or watercress)
• 6 ounces fresh mozzarella cheese, sliced ¼ inch thick
• ⅓ English cucumber, thinly sliced
• 1 medium avocado, sliced
• ⅔ cup alfalfa sprouts