Preheat oven to 400°. In large skillet, heat 2 tablespoons oil over medium-high heat; saute onion until softened. Reduce heat to medium-low; cook until golden brown, 25-30 minutes, stirring occasionally.
Toss tomatoes with 1 tablespoon oil, ¼ teaspoon salt and ⅛ teaspoon pepper; spread in a 15x10x1-in. pan. Roast until softened, 10-15 minutes. Stir tomatoes into onion, mashing lightly. In small bowl, mix cream cheese, blue cheese, garlic and the remaining salt and pepper.
Brush bread slices with remaining oil; grill, covered, over medium heat until lightly toasted, 1-2 minutes per side. Sprinkle steaks with steak seasoning. Grill, covered, over medium heat until meat reaches desired doneness (for medium-rare, a thermometer should read 135°; medium, 140°; medium-well, 145°), 3-5 minutes per side. Let stand 5 minutes before slicing.
To serve, spread toasts with cheese mixture; top with steak and onion mixture. Drizzle with vinegar.
