In a small bowl, mix the Cajun seasoning with olive oil and minced garlic to create a marinade.
Place the chicken breasts on a large sheet pan and rub half of the Cajun marinade all over them.
In a mixing bowl, combine the sliced bell peppers and red onion. Drizzle the remaining marinade over the vegetables and toss to coat.
Arrange the seasoned chicken on one side of the sheet pan and spread the peppers and onions on the other side.
Season everything with salt and pepper to taste.
Roast in the preheated oven for 25-30 minutes, or until the chicken is cooked through and reaches an internal temperature of 165°F (74°C) and the vegetables are tender.
Allow to rest for a few minutes before slicing the chicken.
Sprinkle fresh parsley over the chicken and vegetables for garnish before serving.
Notes
Feel free to adjust the Cajun seasoning to your taste.