Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper for easy cleanup.
Dice the chicken breast, zucchini, yellow squash, bell pepper, and red onion into uniform 1-inch bite-sized pieces to ensure even cooking.
Place the chicken and chopped vegetables (excluding the tomatoes) onto the baking sheet, then drizzle with olive oil and sprinkle with sea salt, black pepper, dried oregano, and garlic powder.
Toss everything thoroughly with your hands or a spatula until well-coated, then spread into a single layer across the pan.
Roast in the oven for 18 to 20 minutes, or until the chicken is cooked through and the vegetables are tender with golden-brown edges.
During the last 2 minutes of roasting, add the diced Roma tomatoes to the pan just to warm them through without letting them fall apart.
Remove from the oven and serve the roasted chicken and vegetable medley over a base of warm, fluffy white rice.