In 10-inch nonstick skillet, heat 2 tablespoons of the oil over medium heat. Add onions; cook, stirring occasionally, 10 to 12 minutes or until deep golden brown. Add vinegar, 1/2 teaspoon of the salt and 1/4 teaspoon of the pepper; cook 1 minute longer or until liquid has evaporated. Remove from heat; set aside.
In small bowl, mix cheese, mayonnaise and remaining 1/4 teaspoon pepper. Set aside.
Heat gas or charcoal grill. Sprinkle steak with remaining 1/2 teaspoon of the salt and remaining 1/2 teaspoon of the pepper. Place steak on grill over medium-high heat. Cover grill; cook for 8- 10 minutes turning once for medium rare or until of desired doneness. Transfer steak to cutting board; let rest 10 minutes.
Lightly brush cut sides of rolls with remaining 2 tablespoons of the oil. Place rolls, cut side down, on grill rack. Cook 1 to 2 minutes or until golden.
Thinly slice steak across grain on the diagonal. Divide steak among bottom halves of rolls; top each with one-fourth of the onions. Spread cut sides of baguette tops with cheese mixture; place on top of onions. Serve immediately.