Line 6 quart or larger slow cooker with a crock-pot liner or spray with a non-stick cooking spray.
Place the sliced onions in the bottom of the slow cooker.
Take out the corned beef from the packaging and discard the seasoning packet included in the package. Rinse off the beef briefly in cold water and place on top of the sliced onions.
Place the bay leaf on top of the corned beef and push the cloves in to the meat all over.
In a small bowl mix together the water, brown sugar, mustard and black pepper and stir to combine.
Pour the mixture carefully over the beef without losing the bay leaf.
Cover and cook on HIGH for 5 to 6 hours.
I pulled it out after the 5 hours, and placed it in a bowl and covered it until we were ready, and then I sliced it.
Notes
For a slightly different flavor profile you could substitute the 3/4 cup of water with your favorite beer or even a little apple juice!