For the fajita seasoning:In a small bowl, whisk together all of the fajita seasoning ingredients.For the chicken and vegetables:Preheat oven to 425°F. Season both sides of chicken breasts with fajita seasoning. Set aside.Arrange onion and peppers evenly on a rimmed sheet pan. Place seasoned chicken on top. Bake until chicken is cooked through and registers 165°F in the largest part, about 22–30 minutes. Remove chicken to a plate and rest for 10 minutes. If veggies aren't cooked enough for your liking, you can bake them more while the chicken rests.Slice chicken and serve with veggies. You can also serve with tortillas, sour cream, avocado, cilantro, and a squeeze of lime.