Preheat your outdoor grill or grill pan to medium-high heat and preheat the oven to 400°F.
Slice the zucchini into rounds, the red bell pepper into strips, and the red onion into wedges.
Toss the prepared vegetables with half of the olive oil, sea salt, and black pepper on a parchment-lined baking sheet.
Roast the vegetables in the oven for 15-20 minutes until they are tender and slightly caramelized.
In a small bowl, whisk together the remaining olive oil, lemon juice, dried oregano, and garlic powder to create the marinade.
Brush the chicken breast thoroughly with the lemon-herb marinade on both sides.
Place the chicken on the hot grill and cook for 6-7 minutes per side, or until the internal temperature reaches 165°F.
Remove the chicken from the grill and let it rest for 5 minutes to ensure the juices redistribute.
Slice the chicken and serve it over the roasted vegetables, garnishing with freshly chopped parsley.