Spread oil on each side of brisket then season both sides generously with salt. “Sear” on a rimmed sheet tray in 550F/287C oven for 10-15 minutes. Flip and continue to cook on the second side for 10-15 minutes. When properly seared, the outside should appear nicely browned.
Add a long squeeze of olive oil into a large fry pan (i’m using a 12” cast iron) over high. Add sliced onions. Cook for 10-12 min over high heat (no salt added) until roasty and slightly caramelized.
Add the roasted onions to a roasting pan and place the seared brisket on top. Add a bit of water to the sheet tray you cooked the brisket on and scrape up any brown bits. Add the resulting liquid into the roasting pan with the brisket and onions.
To make the cooking liquid, whisk together beef stock, better than bouillon, worcestershire, paprika, garlic powder, pepper, thyme, tomato sauce, anchovies, ketchup, and A1. Pour the liquid over the beef in the roasting pan. Cover beef with a piece of parchment, and tightly wrap the entire pan with foil. Bake at 300F/150C for 4 hours.
After 4 hours, remove foil and continue to cook for an additional 1-2 hours to reduce liquid and brown the beef. When done, beef should be tender and almost shreddable, but not falling apart. Allow meat to cool slightly, then refrigerate for 12-24 hours.
Once cooled, remove excess fat from the top of the roasting pan. Remove brisket from cooking liquid and transfer roasting pan to the stove over a medium-high burner to reduce liquid by 2-3 times. Add bloomed gelatin if desired and continue to reduce for another 30 minutes or so to continue to thicken. Taste for seasoning and adjust with salt if needed.
Slice brisket and shingle into roasting pan spooning the sauce on top. Cover with foil and hold in very low oven until ready to serve.