Place the onion on the bottom of the slow cooker. Add the beef fat side up.
Add the seasoning, garlic, bay leaves, lemon juice/apple cider vinegar (optional) and water.
Cook on high for 7 hours or on low for 9-10 hours.
Take the corned beef out of the slow cooker and place on a baking sheet (fat side up). Brush or spray with a little bit of vegetable or olive oil.
Broil the meat for 2-4 minutes, keeping a close eye on it and leaving the oven door open, so it gets crispy and doesn't burn.
Slice and serve with desired side dish.