Heat a large cast iron skillet over medium heat. Add half of the olive oil, and season the chicken thighs very well with salt, pepper, Italian seasoning, oregano, and dried basil, reserving some for the vegetables.
Once skillet is hot, cook the chicken until brown on both sides, about 3-5 minutes per side. Work in batches if needed to avoid over crowding the pan. Once all of the chicken is cooked through and reaches an internal temperature of 165°F, remove from the skillet and set aside.
Add remaining oil to the skillet, then add garlic and cook until fragrant, about 30 seconds.Immediately add onions, bell peppers and tomatoes with remaining seasonings and salt. Cook until everything softens, about 7-10 minutes. Stir in the giardiniera and fresh basil to heat through, reserving some to top the chicken later.
Heat the broiler of your oven on high. Add chicken back to the skillet with the vegetables. Sprinkle the chicken with parmesan cheese, and top with mozzarella slices. Broil for about 3-5 minutes until cheese is bubbly and golden brown. Garnish with fresh basil, more parmesan, and the remaining giardiniera. Enjoy!
