Preheat the oven to 300°F. Look at the raw brisket and locate the direction of the grain. You will want to cut it against the grain when it's cooked.
In a small bowl, mix the salt, pepper, smoked paprika, cumin, onion powder, garlic powder, and dry mustard. Gently rub the spice mix all over the brisket.
Make a foil pouch: Place the brisket, fat side up, on two layers of foil. Pull the edges of the foil up around the brisket and wrap it. Use a third foil layer on the top if needed. Seal well to keep moisture in, but don't wrap the meat too tight; leave some room for the steam.
Place the foil pouch on a foil-lined, broiler-safe, rimmed baking sheet. Cook the brisket for 3 hours (about 1 hour per pound) until very tender.
Turn off the oven and let the meat rest until it's cool enough to handle. Remove the brisket from the oven and open the foil pouch. Pour the cooking juices into a measuring cup. You can skim the fat from the top if you want (I don't).
Turn the oven to broil. Brush the meat with some of the cooking juices. Broil until the fat browns and starts to crisp, for about 2 minutes.
Allow the brisket to cool for a few minutes before slicing. Slice it against the grain and on the bias into ¼-inch slices. Serve it with the cooking juices for dipping.
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Notes
You can use smoked salt for a more pronounced smoky flavor.
Beef brisket comes with a beautiful fat cap. Please don't remove it completely, either before or after cooking. It's the best part! But you can ask your butcher to trim it into a manageable thickness. ¼ inch is ideal.
Cook the brisket fat side up. This will allow some of the melting fat to seep into the meat, making it juicier and more flavorful.
Like other slow-cooked meats, oven brisket improves with time. You can keep the leftovers in the fridge, in an airtight container, for up to 4 days. Slice the leftovers and serve them cold, or reheat them covered in the microwave at 50% power.