Unroll dough; place in ungreased 15x10x1-inch baking pan. Start at center, press out dough to edges of pan.
Bake 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 garlic clove; mix well. Refrigerate.
Heat oil in small saucepan over medium heat until hot. Add onion; cook and stir 2 to 3 minutes or until tender-crisp. 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, boned side up, between 2 pieces of plastic wrap or waxed paper. Working from center, gently pound chicken with flat side of meat mallet or rolling pin until about ¼ inch thick; remove wrap.
Sprinkle chicken with Italian seasoning and garlic.
Spray large skillet with nonstick 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 crisp and heated, turning once. Cut each warm sandwich into quarters.