Trim excess fat from the brisket and if you are using a large one, cut into 2-4 pieces. Heat oven to 300. Season your meat with a little salt and pepper.
Heat the oil over medium high heat in a dutch oven or other oven safe pan large enough to fit all the brisket and liquid. Add the meat, and cook without moving until deeply browned, 3-4 minutes. Flip and brown the other side as well. Transfer to a plate and brown any additional meat, then move that to the plate as well.
Add the mushrooms to the pot along with a pinch of salt and cook, stirring occasionally until they release their liquid and begin to brown. Add the onions and the sugar and stir. Add the flour, garlic, thyme, onion powder, garlic powder and Korean beef powder if using. Stir and cook for a few minutes. Slowly whisk in the beef broth to prevent lumps. Add the bay leaves and bring to a boil.
Cover and transfer to the oven and cook for 3.5-5 hours, until the brisket is falling apart tender. Remove from the oven and remove the bay leaves. Move the meat to a cutting board and cut into slices against the grain. Add the vinegar to the gravy and stir and taste. Add additional salt, pepper or vinegar as desired. Return the cut meat the gravy to reheat.
Serve over mashed potatoes or over slices of white bread.