For the caramelized onions, in a large cast-iron skillet or large cast-iron enamel Dutch oven over medium-low to medium heat, caramelize the onions; melt butter, add onions, season with bay, thyme, salt, pepper and white pepper, cook to golden and soft, 25 minutes, add sherry if using and remove to bowl.
Meanwhile, for the brisket, bring meat to room temperature. Season liberally.
Preheat oven to 325˚F.
Return pan to medium-high heat with oil, 2 turns of the pan. Brown meat 5 minutes on each side, then top with caramelized onions. Add the broth and melt in the demi-glace or bouillon and add Worcestershire. Cover pan with heavy foil or lid and bake 3 to 3 ½ hours until very tender. Slice and serve with potatoes and vegetables of choice.
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