Remove the pan from the oven, uncover it (being careful not to get burned by the steam), and pierce the meat with a fork. If the fork goes in easily and the meat is tender, it's done. If it still seems a bit tough, cover the pan again with the foil and the lid, put it back in the oven and check it again every 15 to 20 minutes. Depending on the cut of meat, the total cooking can take as long as 4 hours or more, so be patient.
When you take the finished brisket out of the oven, uncover it and let it sit in the pan for at least 10 minutes before slicing it.
To serve, transfer the brisket to a cutting board and slice it thinly across the grain. Spoon some of the very well-cooked onions and pan juices on top of each serving. Serve with lightly buttered noodles or mashed potatoes.