Preheat your oven to 400°F and line a large baking sheet with parchment paper for easy cleanup.
Trim the ends off the Brussels sprouts and slice them in half, then chop the red bell pepper and red onion into one-inch pieces.
Place the vegetables on the prepared baking sheet and toss with half of the olive oil, half of the sea salt, and the dried oregano.
Roast the vegetables in the oven for 20 to 25 minutes, tossing halfway through, until the Brussels sprouts are tender and charred on the edges.
While the vegetables roast, season the chicken breast on both sides with the remaining sea salt, black pepper, and garlic powder.
Heat the remaining olive oil in a cast-iron or stainless steel skillet over medium-high heat.
Place the chicken in the hot skillet and sear for 6 to 8 minutes per side, or until the internal temperature reaches 165°F.
Remove the chicken from the pan and let it rest for 5 minutes to lock in the juices before slicing.
Serve the sliced chicken alongside the roasted vegetable medley for a clean and balanced meal.