Season 1 side of beef with salt, pepper & ___. I usually use ground thyme or ground ancho pepper. Use what makes you happy but keep it 'earthy' - this isn't the place for mint or herbes de provence. ;).
Heat a dutch oven to hot, add a couple TB oil & add meat, seasoned side down. Season the top side. When the bottom is nicely seared, flip over. When the other side is nicely seared, remove to a pan with sides to collect juices.
Preheat oven to 300°F.
While the meat is searing, peel, halve & thinly slice the onions.
Add butter & another 2TB oil to pan. When the butter has melted, add onions. Season with 1tsp salt. Stir to scrape up caramelized bits on the bottom. Cook slowly until caramelized, about 15 minutes.
Add can of beef broth & bring to a boil. Stir.
Add meat & any juices that have accumulated in the pan. Cover & put in 300F oven for 2.5 hours or so.
You will not need a knife. The liquid is essentially French onion soup.