Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper for easy cleanup.
Cut the chicken breast into uniform 1-inch bite-sized pieces to ensure even cooking.
Chop the broccoli into small florets and slice the red bell pepper into thin, even strips.
Place the chicken, broccoli, and bell peppers onto the prepared baking sheet.
Drizzle the olive oil over the mixture and sprinkle with sea salt, black pepper, garlic powder, and dried oregano.
Toss everything thoroughly with your hands or tongs until the chicken and vegetables are well-coated in the oil and spices.
Spread the mixture out into a single layer, ensuring nothing is overcrowded so the vegetables roast rather than steam.
Roast in the oven for 18 to 20 minutes, or until the chicken is cooked through and the vegetables are tender with slightly charred edges.
Remove from the oven and immediately drizzle the fresh lemon juice over the hot sheet-pan ingredients to brighten the flavors.
Transfer the roasted mixture to a bowl and top with the fresh avocado slices before serving.