In large skillet, cook and stir onion in 2 tablespoons oil over medium-high heat until softened. Reduce heat to medium-low; cook 30 minutes or until golden brown, stirring occasionally.
Place tomatoes in a shallow baking pan. Add 1 tablespoon oil, 1/4 teaspoon salt and 1/8 teaspoon pepper; toss to coat. Bake at 400° for 10-15 minutes or until softened. Lightly mash tomatoes; stir into onion.
In small bowl, mix cream cheese, blue cheese, garlic and remaining salt and pepper until blended.
Brush both sides of baguette slices with remaining oil. Grill over medium heat 1-2 minutes on each side or until toasted.
Sprinkle both sides of steaks with steak seasoning. Grill, covered, over medium heat for 3-5 minutes on each side or until meat reaches desired doneness (for medium-rare, a thermometer should read 145°; medium, 160°; well-done, 170°). Let stand 5 minutes before slicing.
Spread cheese mixture onto toasts; top with steak and onion mixture. Drizzle with vinegar. Yield: 16 appetizers (3/4 cup tomato jam).