If brisket is packer trimmed, trim off most of excess fat (there can be a lot); leave some on to help tenderize. If it is trimmed already, you may not need to trim any fat.
Mix together rub ingredients. If you don't have a garlic pepper blend, just use garlic powder and black pepper (a little more garlic than pepper). The chipotle powder is from Penzey's spices and provides a wonderful warm smoky flavor. You will need about 1/4 to 1/2 cup rub depending on how large your brisket is. Mix the rub ingredients according to your preferences; I mixed mine until the garlic and chipotle seasonings were the predominate smell. Once the rub is mixed to your liking, use your hands and rub it into the meat, front, back and sides.
At this point you can either cook the meat OR wrap the meat tightly (saran wrap and foil) and refrigerate for up to 24 hours. If you refrigerate the meat, it will have time to absorb the seasonings and have a more complex flavor. If you don't refrigerate it, it will be still be good, just not as strong flavored.
Set oven at 350F. Place brisket in pan large enough to hold it (unwrap it first if you did that step). Place sliced onions on top of brisket, enough to thickly cover top of meat. Cover brisket tightly with foil and cook 4 hours. If brisket is thin, check it at 2 1/2 to 3 hours and see if its tender. Brisket must be cooked long and slow to be tender but you also don't want to dry it out.
Sometime in hour 4, prepare the sauce. Pour the beer in a bowl (you can use a smaller/large amount of beer, I just happen to have a 12 ounce bottle on hand when I made this). Add enough barbecue sauce so that the mixture is not as runny as the beer but not quite as thick as the barbecue sauce (but truthfully, if you make it thicker or thinner, it will be OK, just may have a different flavor), about the thickness of a cake mix would be good. Add a few dashes of Worcestershire sauce (to taste) and some chipotle hot sauce (again to taste, found in the Mexican food section or a Mexican market). Mix together well; a fork will probably be easier and cleaner than a spoon (trust me on this one).
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After the meat has cooked 4 hours, remove from oven. You can double check to make sure its done by inserting a meat thermometer and seeing if it registers 170F. Also, use a fork and pull on the meat to see how tender it is. It should start to shred, if not, you may need to cook it longer. There will probably be a lot of juices in the pan, they will be mostly fat. Drain as much of the fat as you can; it will be OK to leave some but remove most of it. You can now slice the brisket or leave it whole. Leave the onions on top when you do slice it. Pour the sauce all over the brisket, recover with the foil and cook another hour (it will be OK if you need to remove it sooner than an hour). If you didn't slice it before, now is the time. You can drag the slices through the sauce or spoon the sauce over the slices...the sauce will be full of onion bits and some meat and will be really tasty.
IMPORTANT: When you slice the brisket, be sure to slice it at an angle and not straight across, this will help keep the meat tender.
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