Coat a large nonstick skillet with cooking spray; add oil, and place over medium heat until hot. Add sugar; cook until sugar dissolves, stirring constantly. Continue cooking an additional 2 minutes or until golden, stirring constantly. Add onion and 1/4 teaspoon salt (caramelized sugar will harden and stick to spoon, but will melt again). Cook 20 minutes or until onion is golden, stirring occasionally. Add orange juice and bourbon; cook 5 minutes or until liquid evaporates. Remove from heat; set aside, and keep warm.
Trim fat from steak. Combine mustard, 1/4 teaspoon salt, and garlic; stir well. Rub mustard mixture over both sides of steak. Pat pepper into mustard mixture on steak.
Prepare grill or broiler. Place steak on grill rack or broiler pan coated with cooking spray; cook 8 minutes on each side or until desired degree of doneness. Cut diagonally across the grain into thin slices.
Divide steak evenly among bottom halves of buns; top each with 1 1/2 tablespoons onion mixture and top half of bun.