Boneless, skinless chicken breasts or thighs are typically used for fajitas. The chicken is cut into thin strips and cooked until cooked through. Bell peppers, onions, and sometimes mushrooms are the most common vegetables used in fajitas. The vegetables are typically cut into strips and cooked until crisp. Fajitas are typically seasoned with a blend of spices, such as chili powder, cumin, garlic powder, onion powder, and salt.