Preheat oven to 375 degrees Fahrenheit. Unroll dough; place in greased 15-x 10-inch baking pan. Starting at center, press dough to edges of pan.
Bake for 10 minutes. Cool 15 minutes or until completely cooled.
Meanwhile, cook spinach as directed on package. Drain well; squeeze dry with paper towels. In small bowl, combine mayonnaise and 1 of the minced garlic cloves; mix well. Refrigerate.
Heat oil in small saucepan over medium-high heat until hot. Add onion; cook and stir 2 to 3 minutes or until crisp-tender. Add sugar and vinegar. Reduce heat to low; simmer 3 to 5 minutes or until most of liquid has evaporated, stirring occasionally.
To flatten each chicken breast half, place between 2 pieces of plastic wrap or waxed paper, bone side up.
Working from the center, gently pound chicken with flat side of meat mallet or rolling pin until about 1/4 inch thick; remove wrap. Sprinkle chicken with Italian seasoning and remaining minced garlic.
Spray large skillet with non-stick cooking spray. Heat over medium-high heat until hot. Add chicken; cook 8 minutes or until browned, fork-tender and juices run clear, turning once.
Cut cooled pizza crust into 4 rectangles. Remove rectangles from pan; spread each with 1 tablespoon mayonnaise mixture. Top 2 rectangles with chicken, spinach, onion mixture, cheese and remaining crust rectangles, mayonnaise side down.
Heat large skillet or cast iron skillet over medium heat until hot. Place sandwiches in skillet. Place smaller skillet on sandwiches to flatten slightly. Cook about 1 to 2 minutes or until crispy and heated, turning once. Cut each warm sandwich into quarters.