Combine ingredients for herbed mayonnaise and set aside.
Butter outside of each slice of bread and set aside.
Slice tomatoes thinly (to about the same thickness of chicken)
Season chicken with salt and pepper. Heat a drizzle of oil in a medium pan over medium heat and cook chicken until browned and cooked through.1
While chicken is cooking, preheat panini press to medium heat and cook bacon according to package directions.
Remove chicken from pan.
Begin assembling by spreading the inside of both bread slices with some of the herbed mayo.
Next, layer the chicken, tomato, spinach, bacon and top with two slices of cheese.
Place the top on the sandwich and carefully transfer to panini press, one to two sandwiches at a time depending on size of press.
Press down slowly and firmly, then cook until the bread has browned and the cheese is melted. Carefully remove from the press and serve warm.