Preheat the oven to 425°F (220°C). Line a baking sheet with parchment paper.
Cut the chicken breast into bite-sized pieces. In a bowl, lightly whisk the egg whites.
Coat the chicken pieces in egg white and then dust them with cornstarch to form a light crispy layer.
Place the coated chicken on the prepared baking sheet. Bake in the oven for about 15-18 minutes, or until cooked through and lightly crispy.
Meanwhile, chop the red bell pepper, broccoli florets, and carrot into even pieces. Toss the vegetables with olive oil, salt, and pepper.
Spread the vegetables on another baking sheet and roast in the oven for about 15 minutes, until tender and slightly caramelized.
In a small saucepan over medium heat, warm the homemade sweet & sour sauce to ensure it coats the chicken evenly.
Once the chicken is done, remove from the oven and toss gently with the warmed sweet & sour sauce.
Plate the crispy sweet & sour chicken alongside the roasted vegetables and serve immediately.