Prepare Garlic-Basil Mayo: Mix mayonnaise, minced garlic, chopped basil, salt, and pepper in small bowl until well combined. Cover and chill until ready to use, or up to 1 hour (this will allow time for the flavors to develop).
Alternatively, instead of chopping the herbs and garlic, you can combine everything in a food processor until finely chopped, then stir in the mayonnaise and blend until smooth.
Onto two slices of Butter Bread, divide the sprouts, Swiss cheese, turkey, tomato, red onion, and spinach. Season with salt and pepper, to taste.
Spread garlic-basil mayonnaise on the other two slices of Butter Bread and add onto the sandwiches, mayonnaise-side-down. Enjoy!