The soubise is basically a béchamel sauce using an onion base. It is pretty much "fine dining."
— super thin-sliced white onion — melted butter — kosher salt — a bay leaf or two — heavy cream — white pepper — nutmeg — fine salt
Onion slices into narrow (small) pot over low heat in order to fit snuggly. Toss to ensure butter coats all the onion slices. Add salt.
Looking for onions to begin clarifying not browning (zero color) in the least.
After about 20 minutes, remove the bay leaves.
Osterize (or immersion blend) the cooked onions into a puree. Strain through sieve if you want super fine (no left-over large bits of onion).
Cook chosen meat (beef, pork, steak, chop, whatever) in skillet to desired doneness.
Put sauce into skillet or a pan over low heat to reheat and mix in trace amounts of white pepper and nutmeg.
Using a big spoon, garnish the plate with a swirl of sauce, pose meat on it, then re-garnish top of meat with more sauce if desired.