Start by toasting the whole grain bread slices until golden brown for added crunch.
In a small bowl, mash the avocado with a fork and season with salt and pepper.
Spread the mayonnaise on one slice of toasted bread and the Dijon mustard on the other slice.
Layer the mashed avocado on the mayonnaise side of the bread.
Add the sliced turkey breast on top of the avocado, followed by the fresh spinach leaves and tomato slices.
Place the other slice of bread, mustard side down, on top of the sandwich.
Gently press down to secure the layers, then cut the sandwich in half for easier handling.