Green Goddess Chicken Sandwiches

Name: Green Goddess Chicken Sandwiches
Ingredients: 2 chicken breasts, cut crosswise in half and pounded to 1/2-inch thickness; Nonstick cooking spray or neutral oil; 1 teaspoon kosher salt, divided; 1/2 teaspoon ground black pepper; 1 small clove garlic (or 1/2 of a larger clove); 1/2 cup fresh herbs (I used 2 tablespoons each basil, chives, cilantro, and parsley); 1 avocado, peeled and pitted; 7 ounces Greek yogurt; 1 tablespoon honey; 2 teaspoons fresh lemon juice; 2 teaspoons white balsamic vinegar; 1/4 teaspoon ground white pepper; 4 hamburger buns or sandwich rolls; 1 cup fresh baby spinach leaves