Preheat oven to 450 degrees F.
Mince garlic. Trim green beans, Cut cauliflower into florets.
Drain roasted red peppers.
Line a large baking sheet with parchment paper.
Make Cajun blend: In a small bowl, combine garlic powder, paprika, pepper, salt, onion powder, cayenne pepper, oregano, thyme, and red pepper flakes (if using).
In a small bowl, whisk together the olive oil, lemon juice, Cajun spice blend, and garlic. Set aside.
Evenly spread the green beans, cauliflower, and roasted red peppers on the baking sheet. Drizzle 2 tablespoons of marinade over the vegetables and toss until combined. Cook in the oven for 15 minutes.
Add chicken to the marinade and let marinate for 15 minutes.
Remove vegetables from the oven and nestle the pieces of chicken between the vegetables onto the pan. Bake for an additional 15-20 minutes until internal temperature of the chicken reaches 165 degrees F and vegetables are tender.