Heat oven to 450 degrees F. In large nonstick skillet, melt butter over medium to medium-high heat. Add onion and cook, stirring, 5 minutes.
Pour beer and 1 cup of the beef broth into skillet. Stir in sugar, salt and pepper and bring to a simmer. Cook 3 minutes. Pour mixture into an oven-proof baking dish.
Return skillet to medium-high heat. Sprinkle steak with Montreal seasoning. Add to skillet and brown on both sides, 3 minutes total. Add remaining beef broth to skillet, scraping up any brown bits from pan. Transfer steak to baking dish; pour skillet contents over steak.
Cover dish with foil and roast at 450 degrees F for 25 minutes. Remove from oven and increase heat to broil. Open rolls slightly and toast under broiler for 1 to 2 minutes.
Thinly slice steak against the grain and return to baking dish. Divide meat and onions among rolls, spooning sauce over each. Serve sandwiches with additional sauce for dipping alongside.
