Preheat your oven to 400°F and line a large baking sheet with parchment paper for easy cleanup.
Prepare the vegetables by halving the Brussels sprouts, chopping the bell pepper into 1-inch pieces, and slicing the red onion into thick wedges.
Place the vegetables on the baking sheet and toss with 0.5 tbsp of olive oil, 0.25 tsp sea salt, 0.25 tsp garlic powder, and 0.25 tsp dried oregano.
Roast the vegetables in the oven for 20 to 25 minutes, tossing halfway through, until the Brussels sprouts are tender and have developed golden brown edges.
While the vegetables are roasting, season the chicken breast evenly on both sides with the remaining sea salt, black pepper, garlic powder, and dried oregano.
Heat the remaining 0.5 tbsp of olive oil in a heavy-bottomed skillet or cast-iron pan over medium-high heat.
Place the chicken in the hot skillet and sear for 6 to 7 minutes per side, or until the internal temperature reaches 165°F and the surface is deeply browned.
Remove the chicken from the pan and let it rest for 5 minutes to lock in the juices before slicing.
Plate the sliced chicken alongside the roasted vegetables and finish the entire dish with a fresh squeeze of lemon juice.