*If making fresh pesto, combine the following ingredients in a blender or food processor: 3 cups lightly-packed fresh basil leaves, 1/2 cup freshly grated Parmesan, 1/3 cup toasted pine nuts, 3 cloves garlic, 1 teaspoon sea salt, 1/2 teaspoon freshly-cracked black pepper. Pulse until finely chopped, then with the food processor running, slowly drizzle in ½ cup extra-virgin olive oil until mixture is smooth and fully incorporated. Use immediately or refrigerate for up to 3 days.
Combine marinade ingredients in a zip-top freezer bag; add chicken breasts and marinate in fridge for at least 2 hours or overnight.
If Using Immediately: Keep refrigerated until ready to use, up to one day.
If Freezing: Transfer bag to freezer and freeze for up to 3 months. When ready to cook, transfer to refrigerator for 24 hours, or until fully thawed.
To Prepare Chicken: Heat grill or grill pan to medium-high. Grill for 20-22 minutes, flipping halfway through, until cooked through (165°F) and the juices run clear. Remove chicken from heat, loosely cover with foil, and let rest for 5 minutes. Cut chicken breasts in half crosswise into bun-sized portions.
Meanwhile, in a small bowl, whisk together the mayonnaise and lemon juice.
Assemble Sandwich: Top each bun with lemon-mayo, spinach, chicken, tomato, and onion. Serve immediately.