Preheat your oven to 400 degrees Fahrenheit.
Coat the chicken breast with olive oil and season evenly with dried oregano, dried thyme, sea salt, and black pepper.
Place the chicken on a parchment-lined baking sheet and roast for 20 to 25 minutes until the internal temperature reaches 165 degrees Fahrenheit.
Allow the chicken to rest for 5 minutes before slicing it into thin, even strips.
Toast the whole grain bread slices until they are golden and firm.
Spread the Dijon mustard across one side of each toasted bread slice.
Place the baby spinach and tomato slices on the bottom slice of bread.
Arrange the sliced roasted chicken over the vegetables and top with the slice of provolone cheese.
Optional: Place the open sandwich under the broiler for 60 seconds to melt the cheese until bubbly.
Top with the remaining bread slice, cut diagonally, and serve warm.