Preheat oven to 450 degree F. In a large skillet cook onion in butter and oil over medium heat for 10 minutes, stirring occasionally. Stir in sugar, vinegar, and dates. Cook and stir 5 minutes more or until onions are golden brown. Remove from heat; stir in nuts. In a bowl combine cheeses; sprinkle evenly over bread shell. Top with onion mixture. Place bread shell on an ungreased baking sheet. Bake for 8 minutes or until cheese is melted. Let stand 5 minutes before cutting into 2-inch squares. Makes about 30 appetizers.