Philly Cheesesteak Sandwiches

hoagie rolls, split

butter

large white onion, cut in half and thinly sliced

rib-eye or flat iron steak (about 3/4 pounds), sliced thinly across the grain and then roughly chopped*

salt & pepper

butter, for cooking

slices Havarti cheese (Provolone or Muenster would work as well)