Caramelized Onion and Steak Sandwiches

INGREDIENTS

• 2 Tbsp. vegetable oil
• 2 medium onions, thinly sliced, about 4 cups
• 1 cup Hellmann's® or Best Foods® Real Mayonnaise
• 1/2 cup balsamic vinegar
• 12 ounces flank steak or sirloin steak, grilled or broiled, then sliced
• 1 loaf French bread, split and cut into 4 pieces