To start cooking, preheat the oven to broil. Toast the bread for a minute or so, keeping a close eye on it. Once lightly toasted, spread about a teaspoon of the caramelized onions over top. Cover with a slice of brie and set aside while you cook the pork.
To cook the pork, preheat a large, fry pan to medium-high heat. Once hot, add the olive oil and butter and let melt. Lay the pork in a single layer and cook it only for about a minute or so, just until you start to see a bit of white around the edge. Then flip it over and cook for another 30 seconds to a minute. Pork can be served with a tiny bit of pink in the middle. Keep in mind that the pork will continue to cook even after it has been removed from the heat. Transfer to a tray to rest, while you cook the second batch.
While all the pork is resting, place the bread back into the oven for about 30 seconds to a minute, just to slightly melt the cheese.
Lastly, place a piece of pork on top of the cheese, first coating it with the juices that are on the resting tray. The juices will give the meat a nice shine. Sprinkle with a tiny bit of fleur de sel, if desired, and serve immediately.