Preheat your oven to 400°F (200°C) and line a large sheet pan with parchment paper for easy cleanup.
Cut the chicken breast into 1-inch bite-sized cubes and chop the zucchini and bell peppers into similar sizes to ensure even cooking.
Place the chicken, zucchini, bell peppers, and the frozen vegetable medley with potatoes onto the prepared sheet pan.
Drizzle the olive oil and lemon juice over the chicken and vegetables, then sprinkle evenly with dried oregano, garlic powder, sea salt, and black pepper.
Toss everything thoroughly with your hands or tongs until well coated, then spread into a single, even layer across the pan.
Roast in the center of the oven for 18 to 20 minutes, or until the chicken is cooked through and the vegetables are tender and slightly caramelized.
While the sheet pan is roasting, warm your pre-cooked white rice in a small saucepan or microwave.
Divide the rice into a bowl and top with the roasted chicken and vegetable mixture, drizzling any remaining pan juices over the top before serving.