1 4-lb. piece flat-cut beef brisket, untrimmed
Kosher salt
1/4 cup Dijon mustard
1/4 cup (packed) dark brown sugar
1 tablespoon grated peeled ginger
2 tablespoons bacon fat or vegetable oil
2 medium yellow onions, thinly sliced
1/4 cup all-purpose flour
1 bay leaf
1 750-ml bottle Belgian-style tripel ale
4 cups beef stock or low-sodium chicken broth
Season brisket with salt. Wrap tightly in plastic and chill at least 8 hours.
Let brisket sit at room temperature 1 hour.
Preheat oven to 400°. Combine mustard, brown sugar, and ginger in a small bowl. Unwrap brisket, place on a wire rack set inside a large rimmed baking sheet, and rub mustard mixture all over brisket. Roast until top is nicely browned, 30–40 minutes.
Remove brisket from oven and reduce oven temperature to 300°.
Meanwhile, heat bacon fat in a large heavy ovenproof pot over medium heat. Add onions; season with salt and cook, stirring often, until deep golden brown, 8–10 minutes. Reduce heat to medium-low, add flour, and cook, stirring often, until mixture smells nutty, about 4 minutes.
Add bay leaf, ale, and stock. Bring to a simmer. Add brisket, cover, and transfer to oven. Braise, turning every 30 minutes, until fork-tender, 3–4 hours. Discard bay leaf.
Transfer brisket to a cutting board and let rest at least 20 minutes.
If braising liquid is thin, bring to a boil, reduce, and simmer, skimming surface as needed, until thick enough to coat a spoon; season with salt, if needed.
Slice brisket against the grain. Serve with braising liquid.
DO AHEAD: Brisket can be braised 2 days ahead. Cover and chill in braising liquid.