Season brisket with salt. Wrap tightly in plastic and chill at least 8 hours. (I did not do this. In all honesty, I forgot about this step for three days in a row, then started to worry about the usability of the meat, so I just skipped it today.)

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 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 (forgot to do this too, don't make more work for yourself), 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.