Preheat your oven to 400°F and line a large rimmed baking sheet with parchment paper for easy cleanup.
Dice the chicken breast into one-inch cubes and chop the zucchini, red bell pepper, and red onion into similarly sized pieces.
In a large mixing bowl, whisk together the olive oil, lemon juice, dried oregano, sea salt, and black pepper to create the marinade.
Add the chicken and chopped vegetables to the bowl, tossing thoroughly until every piece is evenly coated with the herb mixture.
Spread the mixture across the prepared baking sheet in a single layer, ensuring the pieces are not overcrowded to allow for proper roasting.
Roast in the center of the oven for 18 to 22 minutes, or until the chicken is cooked through and the vegetables are tender with slightly charred edges.
Remove from the oven and garnish with freshly chopped parsley to add a bright, clean finish before serving.