Freeze the sirloin for 30 minutes and slice it paper-thin across the grain.
Heat the oil on a griddle over medium-high heat and cook the onion with 1/4 teaspoon salt for 8 minutes until deeply golden, then transfer it to a plate.
Toss the shredded cheddar with the cornstarch in a small saucepan, add the evaporated milk and garlic powder, and stir over low heat until smooth and thick enough to coat a spoon.
Spread the beef on the hot griddle, season with the remaining salt and black pepper, and cook for 3 to 4 minutes until browned and at least 145°F.
Fold the fried onion into the beef and divide the mixture among the rolls.
Spoon 3 to 4 tablespoons warm cheese sauce over each sandwich and serve before the sauce tightens.