Halve and peel the onions lengthwise, then slice lengthwise (end to end) about ¼ inch thick.
Pat beef dry and tie with kitchen string, about 1½ to 2 inch intervals. Generously season beef all over with kosher salt and 1 teaspoon of fresh cracked pepper. Heat 1 tablespoon oil in large heavy pot until it shimmers. Brown beef on all sides, about 15 minutes. Brown well, you want a good crust, not gray meat, this gives the dish tons of flavor. When done browning, transfer to a plate.
Preheat oven to 350º.
Add remaining tablespoon oil to pot, then the onions and bay leaves. Add a generous sprinkle of salt and cook onions until well browned, scraping up any browned bits from bottom of pan and stirring occasionally, about 20 - 25 minutes. Watch closely, if bottom is getting too dark and starting to burn, deglaze and scrape up the fond with a splash of beer. That's what I did and then the bits came right up.
After onions have browned, add the beer and vinegar to onions and bring to a boil. Again, stir and scrape up any browned bits. Add the beef and any accumulated juices and return to a boil.
Cut a round of parchment paper the diameter of the inside of pot, I traced the lid and then cut out the round. Place parchment round on top of meat and onions.
Cover, and braise in oven until meat is very tender, about 3 to 3½ hours.
Transfer beef to cutting board and tent loosely with foil, let rest 20 minutes. Remove string and then slice meat. Skim fat off surface of sauce and onions and discard bay leaves. Serve braised beef with onions and sauce, and mashed potatoes or egg noodles.
Alternately, make at least one day in advance, and when beef has cooled, chill in sauce in refrigerator overnight. Discard solidified fat (a little is okay, flavor!). Remove meat from sauce and slice, layer in shallow baking dish and spoon gelled sauce and onions over the sliced meat. Cover tightly with foil and reheat in 325º oven for 45 minutes.