1Heat oven to 350°F. In 10-inch skillet, heat oil and butter over medium heat. Add onion; cook 8 to 10 minutes, stirring frequently, until onion is softened and light golden brown.
2On ungreased cookie sheet, place focaccia. Sprinkle 3/4 cup cheese on focaccia. Top with onion and tomatoes; sprinkle with remaining 1/4 cup cheese.
3Bake 10 to 15 minutes or until cheese is melted and bread is hot. Sprinkle with arugula.