Onion-Braised Beef Brisket

1. Adjust oven rack to lower-middle position and heat oven to 300 degrees. Line 13 by 9-inch baking dish with two 24-inch-long sheets of 18-inch-wide heavy-duty foil, positioning sheets perpendicular to each other and allowing excess foil to extend beyond edges of pan. Pat brisket dry with paper towels. Place brisket fat side up on cutting board; using dinner fork, poke holes in meat through fat layer about 1 inch apart. Season both sides of brisket with salt and pepper.
2. Heat 1 teaspoon oil in 12-inch skillet over medium-high heat until oil just begins to smoke. Place brisket, fat side up, in skillet (brisket may climb up sides of pan); weight brisket with heavy Dutch oven or cast-iron skillet and cook until well browned, about 7 minutes. Remove Dutch oven; using tongs, flip brisket and cook on second side without weight until well browned, about 7 minutes longer. Transfer brisket to platter.
3. Pour off all but 1 tablespoon fat from pan (or, if brisket was lean, add enough oil to fat in skillet to equal 1 tablespoon); stir in onions, sugar, and 1⁄4 teaspoon salt and cook over medium-high heat, stirring occasionally, until onions are softened, 10 to 12 minutes. Add garlic and cook, stirring frequently, until fragrant, about 1 minute; add tomato paste and cook, stirring to combine, until paste darkens, about 2 minutes. Add paprika and cayenne and cook, stirring constantly, until fragrant, about 1 minute. Add flour and cook, stirring constantly, until well combined, about 2 minutes. Add broth, wine, bay leaves, and thyme, stirring to scrape up browned bits from pan; bring to simmer and simmer for 5 minutes to fully thicken.
4. Pour sauce and onions into foil-lined baking dish. Nestle brisket, fat side up, in sauce and onions. Fold foil extensions over and seal (do not tightly crimp foil because foil must later be opened to test for doneness). Place in oven and cook until fork slips easily in and out of meat, 3-1⁄2 to 4 hours (when testing for doneness, open foil with caution as contents will be steaming). Carefully open foil and let brisket cool at room temperature, 20 to 30 minutes.
5. Transfer brisket to large bowl; set fine-mesh strainer over bowl and strain sauce over brisket. Discard bay leaves and thyme from onions and transfer onions to small bowl. Cover both bowls with plastic wrap, cut vents in plastic, and refrigerate overnight.
6. About 45 minutes before serving, adjust oven rack to lower-middle position; heat oven to 350 degrees. While oven heats, transfer cold brisket to carving board. Scrape off and discard any fat from surface of sauce, then heat sauce in medium saucepan over medium heat until warm, skimming any fat on surface with wide shallow spoon (you should have about 2 cups sauce without onions; if necessary, simmer sauce over medium-high heat until reduced to 2 cups). Slice brisket against grain into 1⁄4-inch-thick slices and place slices in 13 by 9-inch baking dish. Stir reserved onions and vinegar into warmed sauce and season with salt and pepper to taste. Pour sauce over brisket slices, cover baking dish with foil, and bake until heated through, 25 to 30 minutes. Serve immediately.

SAME-DAY ONION-BRAISED BEEF BRISKET
After removing brisket from oven in step 4, reseal foil and let brisket sit at room temperature for 1 hour. Transfer brisket to carving board and continue with straining, defatting, and reheating sauce and slicing meat; omit step of returning brisket to oven once reheated sauce is poured over it.