Pat chicken breasts dry with paper towels. For even cooking, either butterfly thick breasts or pound to an even 3/4-inch thickness.
Season chicken with 3/4 teaspoon salt. Place on a wire rack over a baking sheet and refrigerate uncovered for 1-2 hours for dry brining (optional but recommended).
In a small bowl, combine garlic powder, onion powder, smoked paprika, cumin, oregano, cayenne (if using), remaining salt, and black pepper to create the seasoning mixture.
Remove chicken from refrigerator 20 minutes before cooking. Rub chicken with 1 tablespoon olive oil, then coat thoroughly with the seasoning mixture.
Heat a cast iron skillet over medium-high heat. Add remaining olive oil. When oil shimmers, add chicken breasts presentation side down. Cook undisturbed for 5-7 minutes until a golden crust forms.
Flip chicken, reduce heat to medium, and continue cooking for 5-7 minutes or until internal temperature reaches 160°F (71°C) at the thickest part.
Transfer chicken to a plate and loosely tent with foil. Rest for 5 minutes to allow juices to redistribute.
In the same pan, add prepared low-carb vegetables and tomatoes. Sauté for 3-4 minutes until vegetables begin to soften but retain crispness.
Add chicken broth and apple cider vinegar, scraping up browned bits from the pan to create a flavorful sauce.
Slice chicken against the grain. Plate with vegetables and spoon pan sauce over chicken. Garnish with fresh herbs.