Pound or slice the chicken breasts to an even thickness.
Combine the marinade ingredients: tequila, grapefruit juice, lime juice, cumin, chili powder, olive oil and garlic powder in a zip top bag then add the chicken, moving around to coat. Marinate 1 hour up to overnight for best flavor.
Drain the marinade, shaking any excess from the chicken then season generously with salt and pepper.
Add 1 to 1 1/2 tablespoons of olive or vegetable oil to a large skillet over medium heat. Once hot, add the chicken. Cook 5 minutes on the first side (without touching them) then turn and continue cooking another 3-4 minutes or until no longer pink.
Transfer the chicken to a plate and tent with foil.
Add the sliced onion and peppers to the same skillet. Season with salt and pepper and cook over medium heat until they're tender and the edges starting to brown.
Slice the chicken then return it to the skillet with the vegetables. Combine and simmer 2-3 minutes.
Serve with tortillas and your favorite fajita toppings.