Preheat your oven to 400°F (200°C).
Rub the chicken breast with olive oil, dried thyme, dried rosemary, garlic powder, sea salt, and black pepper until evenly coated.
Place the chicken on a parchment-lined baking sheet and roast for 20-25 minutes, or until the internal temperature reaches 165°F.
Remove the chicken from the oven and let it rest for 5 minutes before slicing it into thin strips.
While the chicken rests, toast the sprouted grain bread slices until they are golden and firm.
In a small bowl, mash the avocado with a pinch of salt and spread it onto one slice of the toasted bread.
Spread the Dijon mustard onto the other slice of toasted bread.
Assemble the sandwich by layering the arugula, tomato slices, red onion, and the sliced herb-roasted chicken between the bread slices.