Preheat oven to 350 degrees.
Combine beer, ketchup, and cranberry sauce in a medium bowl and set aside.
Add the olive oil to the roasting pan on the stovetop, preheat the pan to medium or medium high. Add the sliced onions. As they begin to cook, salt and pepper your brisket.
Once the onions are starting to brown, push them to the side of the pan and add the brisket in fat side down. Let it brown thoroughly on the first side.
Then turn the brisket over to brown on the other side.
Turn the burner off and pour the braising liquid over the meat. Cover the pan with tin foil (or a cover if you have one - for example if you're using a Le Creuset or similar braising pot). Place in the oven and set the timer for 2 hours.
After 2 hours, take brisket out of the oven and place the brisket on a large cutting board. Let it cool down for a few minutes, then slice it against the grain. (crucial to cut against the grain or it will be tough to chew even if it is cooked properly)
If you would like to cook carrots and potatoes with the brisket (yummy!), add them to the pan, then place the sliced brisket back in as well. Cook for about 1 more hour. Size of brisket, type of pan, and oven will all affect the actual cooking time. However, you can’t really over cook it and if you check it with a fork and it is easy to grab a piece of a slice, you know you’re good to go. Also, check the carrots and potatoes if you added them to be sure they are fork tender.
You can now either serve the brisket immediately or let it cool uncovered for about half an hour and refrigerate it to eat up to 2 days later. You can also freeze the brisket for later use.