In a small bowl, combine the seasonings. Set aside (Note 2)
In the bottom of the crock pot (slow cooker), place the sliced onions.
Remove the brisket from the packaging and rinse under cold water. (Note 3) Rub the spice mixture over the brisket, covering it on all sides. Place it on top of the onions.
Cover and cook on low for 6-8 hours until the meat is fork-tender. The actual cooking time will vary based on the actual size of the brisket and the slow cooker used. If you can pull off a corner of the meat without a knife, it's done. It's important to cook until the temperature reaches 160 degrees.
Remove to a cutting board to cool slightly before slicing. Slice against the grain.
Serve with roasted potatoes and carrots. Store leftovers in an airtight container in the refrigerator.
Be sure to use corned beef brisket. Brisket that has been brined will be labeled "corned beef brisket".
Alternatively, you can use the spice packet if one was included with the corned beef.
Rinsing the corned beef helps to control the salt level since the corned beef is cured in a brine. (This is optional.)
Always cook corned beef on the low setting of the slow cooker.
Corned beef will still have a pink hue when fully cooked. The best test for doneness is to check if the meat is fork-tender. It should pull apart without a knife.