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1
In a large skillet over medium-high heat, heat oil until shimmering. Saute onions, brown sugar, and 1-4 teaspoon salt (to taste) until onions are golden, 10 to 12 minutes. Clear a space in the middle of the pan. Add tomato paste and flour to open space and cook, stirring constantly, until darkened, about 2 minutes. Add garlic and cook until fragrant, about 30 seconds. Stir in broth and cook until sauce thickens, about 4 minutes. Off heat, stir in 2 tablespoons vinegar and transfer mixture to bowl. When cool, cover tightly with plastic and refrigerate.
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2
Whisk together 1 teaspoon salt, paprika, onion powder, garlic powder, and cayenne in a small bowl. Prick brisket with fork, evenly all over both faces. Rub spice mixture over brisket and wrap tightly in plastic. Refrigerate brisket and onion mixture overnight.
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3
The next morning, add half of onion mixture to slow-cooker. Add thyme and bay leaves and place brisket, fat side up, on top. Spread remaining onion mixture over brisket. Cover slow cooker and cook on low until brisket is fork-tender, 9 to 10 hours (or cook on high for 5 to 6 hours). If brisket it especially thick, cook an extra hour. Turn cooker off and allow brisket to rest for 30 minutes.
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4
Remove brisket to cutting board. Cut across grain into 1-to-2-inch slices, and transfer to serving platter. Tent with foil.
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5
Pour sauce into large skillet, discard herbs, and simmer over high heat until slightly thickened, 8 to 10 minutes. Skim off as much fat as possible, add remaining vinegar, then pour half of sauce over brisket. Serve with remaining sauce on side.
