Put the ribeye in the freezer for an hour before you start, this will make it easier to cut into thin slices later.

Slice the onion into thin rings.

Melt half the butter in a skillet and sauté the onions over low heat until golden brown. This takes a long time, at least 15 to 20 minutes.

Cut the ribeye into thin slices. Slice the pistolets open and fry them in the pan until golden brown. Heat the pan, melt the remaining butter, and brown the ribeye in a few minutes.

Make two mounds of the ribeye in the pan, add the onion rings and then the cheese slices.

When the cheese is melted, scrape it up with a large spatula and place it in the bun.