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 1-inch bite-sized cubes and place them into a large mixing bowl.
Prepare the vegetables by cutting the broccoli into small florets, slicing the red bell pepper into strips, and cutting the red onion into thick wedges.
In a small mixing bowl, whisk together the harissa paste, extra virgin olive oil, lemon juice, minced garlic, sea salt, and black pepper until well combined.
Add the vegetables to the bowl with the chicken, then pour the harissa marinade over the top.
Toss everything thoroughly with your hands or a large spoon until every piece of chicken and vegetable is evenly coated in the spicy sauce.
Spread the mixture out in a single layer on the prepared baking sheet, ensuring there is enough space for the air to circulate.
Roast in the center of the oven for 20-25 minutes, tossing the ingredients halfway through, until the chicken is cooked through and the vegetables are tender with charred edges.
Remove from the oven and let rest for 2 minutes before serving to allow the juices to redistribute.