In a small bowl, mix the pickling spices, salt, pepper, and garlic powder. Set aside.
Place the corned beef brisket into your slow cooker. Sprinkle the spice blend all over the top. Arrange the potatoes, carrots, and onion around the brisket, along with the thyme sprigs. Pour in the beef broth.
Cover and cook on LOW for 5 hours.
Remove the lid and add the cabbage to the slow cooker. Cover and cook for another 45-60 minutes or until the brisket reaches an internal temperature of 190 degrees F. (see note)
Remove the brisket to a cutting board and allow to rest for 5 minutes before slicing.
Arrange the sliced brisket on a serving platter and place the veggies around the meat. Serve with whole grain mustard, if desired.
Place the butter, parsley, garlic, and salt in a small microwave-safe bowl. Heat for 30 seconds to 1 minute until melted and stir together. Pour the garlic butter over the veggies. (You will have extra butter.)
Notes
You can exchange 1 cup of beef broth with 1 cup of Guinness if you want a full Irish experience.
Always slice corned beef against the grain to make sure it's tender and not chewy.
With slow cooking, the brisket will be more tender at or around 190 degrees F but is considered fully cooked at 165 degrees F.