Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper for easy cleanup.
Peel the sweet potatoes and cut them into 1/2-inch cubes. Slice the zucchini into half-moons and the red bell pepper into 1-inch strips.
Cut the chicken breast into bite-sized pieces to ensure even cooking and maximum surface area for the spice rub.
In a large mixing bowl, whisk together 2 tablespoons of olive oil, the harissa paste, garlic powder, smoked paprika, sea salt, and black pepper.
Add the chicken, sweet potatoes, zucchini, and bell pepper to the bowl, tossing thoroughly until every piece is coated in the spicy harissa mixture.
Spread the chicken and vegetables in a single layer on the prepared baking sheet, ensuring they aren't crowded so they roast rather than steam.
Roast in the oven for 20-25 minutes, or until the chicken is cooked through and the sweet potatoes are tender with caramelized edges.
While the tray roasts, rinse the quinoa in a fine-mesh strainer. Combine with 1 cup of water in a small pot, bring to a boil, then reduce to a simmer, cover, and cook for 15 minutes until fluffy.
In a small bowl, mash the avocado with the lemon juice and the remaining 0.5 tablespoon of olive oil to create a creamy topping.
Assemble the plate by placing a generous base of quinoa, topping it with the roasted chicken and vegetable mixture, and finishing with a dollop of the lemon-avocado mash.