The prep work for this party appetizer can be done in advance. As guests arrive, simply warm the caramelized onion- and tomato-topped brie in the oven for 12 minutes.
Drain oil from tomatoes; reserve 3 tablespoons. Coarsely chop enough tomatoes for 1/2 cup.
In a large skillet, cook onion in 2 tablespoons reserved oil until tender. Stir in garlic; cook and stir for 1 minute. Add chopped tomatoes and basil; cook for 2 minutes. Remove from heat; stir in pepper.
Line a baking sheet with foil; place Brie on foil. Brush with remaining 1 tablespoon oil. Sprinkle with parsley. Spread tomato mixture over tops. Cover and chill for 30 minutes or up to 5 hours.
Preheat oven to 350 degrees F. Bake Brie on center oven shelf for 12 to 15 minutes or just until edges melt. Transfer Brie to a serving plate. Serve with bread slices. Makes 20 servings.