Preheat the oven to 275°F.

Cut the onion into 1-inch thick wedges. Quarter the mushrooms. Remove the garlic cloves from their skins and smash with the back of a knife.

Set a large Dutch oven or oven-safe pot over high heat and add the olive oil.

Place the beef on a large plate and coat on all sides with salt, black pepper, and brown sugar.

Place the beef in the pot. Sear without moving until browned, 2 minutes. Flip the beef and sear an additional 2 minutes on the opposite side.

Remove the beef from the pot and transfer to a large plate.

Reduce the heat to medium-high and add the olive oil to the pot. Add the onions, mushrooms, and garlic to the pot and cook, stirring occasionally, until lightly browned, 3-4 minutes.

Slowly pour the beer into the pot, gently scraping up any browned bits from the bottom. Return the pot roast to the pot and add the thyme sprigs.

Cover the pot and remove from the heat.

Roast on middle rack of oven until the meat is extremely tender and easily shredded with a fork, 3 1/2-4 hours.

Check to see that pot roast is done. Remove from oven or add time as needed.

Transfer the pot roast to a large serving dish. Shred the meat with two forks. Use a slotted spoon to remove the mushrooms from the pot, and arrange over the pot roast. Serve immediately.