Preheat your oven to 425°F (220°C) and line a baking sheet with parchment paper.
Slice the chicken breast horizontally to form a thinner cut or pound it to an even thickness for faster, even cooking.
In a shallow dish, combine the rolled oats, salt, pepper, and any additional preferred dried herbs or spices.
In another bowl, lightly beat the egg white.
Dip the chicken first in the egg white, then coat it thoroughly in the oat mixture, pressing gently so the coating sticks.
Place the coated chicken on the baking sheet and bake for 20-25 minutes, flipping halfway through, until the chicken is cooked through and the coating turns golden and crispy.
While the chicken is baking, lightly toast the whole wheat bun.
Assemble the sandwich by spreading Dijon mustard on the bun, layering with lettuce, tomato slices, and the crispy baked chicken.
Serve immediately and enjoy your healthy, crispy chicken sandwich.