Heat the oil in a large nonstick skillet over medium-high heat. Add onion; cook 5 minutes, stirring occasionally. Continue cooking 12 minutes or until golden brown, stirring constantly. Stir in vinegar and pepper. Remove from pan. Add spinach to pan; cook 1 minute or until spinach wilts. Remove from heat.
Divide cheese evenly between bottom halves of focaccia. Top each half with 1/2 cup onion mixture, 1/3 cup spinach, and top halves of focaccia. Serve immediately.