Preheat oven to 350°.
Heat 1/2 teaspoon oil in a large nonstick skillet coated with cooking spray over medium heat. Add garlic; sauté 2 minutes. Stir in spinach; cook 4 minutes or until spinach wilts and liquid is absorbed.
Cut focaccia in half horizontally using a serrated knife; place bottom layer, cut side up, in a 12-inch ovenproof skillet. Arrange spinach mixture, bell pepper, chicken, and cheese evenly over bottom half of focaccia.
Combine remaining 2 1/2 teaspoons olive oil, vinegar, salt, and pepper in a small bowl; stir well with a whisk. Drizzle oil mixture evenly over cut side of focaccia top. Cover sandwich with top half of focaccia. Cover pan with foil.
Place a 10-inch cast iron skillet on top of foil; wrap handle of pan with foil. Bake at 350° for 15 minutes or until cheese melts. Cut sandwich into 6 wedges.