Heat oil in Dutch oven or deep 12-inch skillet over medium heat until hot. Brown brisket evenly. Remove brisket; pour off drippings.
Add onions to Dutch oven; cook and stir 3 minutes or until crisp-tender. Place brisket on rack in Dutch oven. Add beer, bring to a boil. Reduce heat; cover tightly and simmer 3-3 1/2 hours or until brisket is fork tender. Remove brisket, keep warm.
Cook cooking liquid over medium-high heat for 5 minutes or until reduced by half. Stir in horseradish.
Trim fat from brisket; carve diagonally across the grain into thin slices. Serve with sauce.
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