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1
Heat 1 1/2 tablespoons oil in a large heavy pot over medium-high heat. Season beef with kosher salt; cook, turning, until browned on all sides, about 15 minutes. Transfer beef to a plate.
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2
Add remaining 1 1/2 tablespoons oil to pot. Add onions, garlic, thyme sprig, and bay leaf. Cook, stirring often, until onions are golden brown and soft, 10-15 minutes. Remove pot from heat and add wine. Return pot to heat, bring to a boil, and simmer until wine is reduced by half, about 3 minutes.
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3
Return beef to pot; add broth. Bring to a boil; reduce heat, cover, and simmer gently until beef is fork-tender, 5-6 hours.
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4
Let beef cool in cooking liquid, then transfer to a shallow baking dish. Strain braising liquid through a fine-mesh sieve over meat in dish. Cover and chill overnight. DO AHEAD:
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5
Cut beef into 4 pieces; set aside. Transfer chilled braising liquid to a large skillet and bring to a boil; cook until thickened and reduced to 1 1/2 cups, 25-30 minutes. Taste and season sauce with salt, if needed.
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6
Add beef to skillet with sauce, cover, and cook over medium-low heat until beef is heated through, 8-10 minutes. Divide beef among plates. Spoon sauce over, top with mushrooms, and sprinkle with flaky sea salt.
