Season the chicken breast evenly on both sides with sea salt, black pepper, and garlic powder.
Heat the extra virgin olive oil in a grill pan or cast-iron skillet over medium-high heat.
Place the chicken in the pan and grill for 5 to 6 minutes per side, or until the internal temperature reaches 165°F.
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.
Slice the grilled chicken breast into thick strips.
Layer the arugula, tomato slices, and grilled chicken onto the avocado-topped bread.
Close the sandwich with the mustard-topped slice, cut in half, and serve immediately.