The focaccia for this sandwich is made from frozen bread dough. For a shortcut, use prepared focaccia or another type of sandwich bread.
To prepare focaccia, thaw dough in refrigerator 12 hours.
Heat 2 teaspoons oil in a small nonstick skillet over medium heat; add onions. Sauté 5 minutes or until soft, stirring often. Stir in oregano; set aside and cool to room temperature.
Combine cornmeal and 2 tablespoons flour; sprinkle cornmeal mixture over clean work surface. Turn dough out onto coated surface. Knead in 2 tablespoons onion mixture and cornmeal mixture for 6 minutes (hard kneading is necessary to soften the elastic in frozen dough); cover with a clean towel and let rest 5 minutes. Roll dough into a 9 x 13-inch rectangle; use remaining flour as needed to keep dough from sticking to work surface. Coat bottom of a 9 x 13-inch baking pan with 1/2 teaspoon oil; place dough in pan, stretching sides to cover bottom. Sprinkle remaining 1 teaspoon oil, remaining onion mixture, and salt evenly over dough. Cover and let rise in a warm place (85°, 1 hour, or until doubled in size).
Preheat oven to 400°.
Bake at 400° for 20 minutes or until underside of bread sounds hollow when tapped. Cool on a rack to room temperature. Cut focaccia in half lengthwise; cut each half into three equal pieces (you should have 6 pieces). Cut each piece in half horizontally.
To prepare chicken, combine basil, salt, and pepper; sprinkle evenly over chicken. Heat 1/2 teaspoon oil in a large nonstick skillet over medium-high heat; add chicken. Cook 4 to 6 minutes per side or until thoroughly cooked; cool completely. Cut chicken into thin diagonal strips.
To prepare mayonnaise, combine roasted peppers and next 4 ingredients (through garlic) in a blender; blend until smooth.
To prepare sandwich, spread 1 tablespoon mayonnaise on bottom half of each focaccia piece. Divide lettuce leaves, tomato slices, chicken, and bacon evenly among sandwich bottoms. Cover with top half of focaccia.