Begin by toasting the whole grain bread slices until golden brown.
In a small bowl, mix the mayonnaise and Dijon mustard together until well combined.
Spread the mayonnaise mixture evenly on one side of each slice of toasted bread.
Layer the sliced turkey breast on one slice of bread, followed by the cooked turkey bacon.
Add the sliced avocado, fresh spinach leaves, and tomato slices on top of the turkey bacon.
Season with salt and pepper to taste.
Place the second slice of bread on top, mayonnaise side down, to complete the sandwich.
Cut the sandwich in half and serve immediately.