Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper.
Cut the chicken breast into 1-inch cubes and halve the Brussels sprouts.
On the baking sheet, toss the chicken, broccoli florets, and Brussels sprouts with the olive oil, sea salt, black pepper, and garlic powder until evenly coated.
Spread the mixture in a single layer and roast for 20-22 minutes, or until the chicken is cooked through and the vegetables are tender with charred edges.
While the vegetables roast, place the riced cauliflower in a non-stick skillet over medium heat and sauté for 5-7 minutes until softened and moisture has evaporated.
In a small bowl, mash the avocado and whisk it together with the Greek yogurt and lemon juice until the consistency is smooth and creamy.
Divide the riced cauliflower into bowls, top with the roasted chicken and vegetable mixture, and finish with a generous drizzle of the avocado-yogurt sauce.