In a large bowl, whisk together the olive oil, balsamic, minced garlic, salt, pepper, and Italian seasoning for the marinade. Add the chicken thighs and move around to coat. Cover and marinate for at least 1 hour, up to 24 hours. (Place in the fridge if it will be longer than 1 hour.)
Heat olive oil in a large skillet over medium-high heat. Shake off excess marinade from the chicken thighs and place them in a single layer on the skillet. Sear for 5-6 minutes per side (reduce heat if the chicken browns too much), until internal temperature reaches 165 degrees F (74 degrees C).
Top the chicken with tomatoes, olives, onions, crumbled feta, and basil ribbons, in that order.