Coat a large nonstick skillet with cooking spray; place over medium-high heat until hot. Add onion; saute 5 minutes. Reduce heat to medium-low; cook 20 to 25 minutes or until onion is golden. Stir in curry powder and garlic; cook, stirring constantly, 5 additional minutes.
Cut a 1/2-inch-thick slice off top of each roll; set tops aside. Hollow out centers of rolls, leaving 1/2-inch-thick shells. Reserve insides of rolls for another use.
Combine mayonnaise and mustard, stirring well. Spread 1 tablespoon mayonnaise mixture over bottom of hollowed out portion of each roll; top evenly with beef. Spoon onion mixture over beef; cover with tops of rolls. Serve immediately.