Preheat your oven to 425°F and line a large rimmed baking sheet with parchment paper for easy cleanup.
Slice the chicken breast into 1-inch bite-sized cubes and place them into a large mixing bowl.
Add the cauliflower florets, bell pepper strips, and red onion wedges to the bowl with the chicken.
In a small ramekin, whisk together the olive oil, harissa paste, lemon juice, sea salt, and black pepper until well combined.
Pour the harissa marinade over the chicken and vegetables, tossing thoroughly with your hands or tongs to ensure every piece is evenly coated.
Spread the mixture in a single, even layer on the prepared baking sheet, ensuring the pieces are not overcrowded.
Roast in the oven for 20-25 minutes, tossing halfway through, until the chicken is cooked through and the vegetables are tender with slightly charred edges.
Remove from the oven and garnish with freshly chopped parsley before serving hot.