Lightly coat a 7- to 8-quart slow cooker with cooking spray.
Remove the corned beef from its packaging and save the seasoning packet. Pat the brisket dry with paper towels.
Spread the onion quarters across the bottom of the slow cooker. Place the corned beef on top, fat side up. Evenly sprinkle the seasoning packet over the meat.
Add the bay leaves and crushed garlic over the brisket. Arrange the carrots and potatoes around the meat.
In a measuring cup, whisk together the broth and Dijon mustard. Pour the mixture into the slow cooker around the meat and vegetables. Add 1 tablespoon of butter to the liquid.
Cover and cook on low for 7 hours.
Add the cabbage wedges, gently pressing them into the cooking liquid. Cover and cook on low for another 2 to 3 hours, or until the meat is very tender and slices easily.
Carefully transfer the meat to a cutting board and let it rest for 15 minutes. Slice thinly against the grain.
To make the garlic herb butter, combine the butter, minced garlic, and parsley in a small bowl. Heat in short intervals in the microwave, stirring between each one, until melted and fragrant.
Drizzle the garlic herb butter over the sliced Easy Slow Cooker Corned Beef with Cabbage and vegetables before serving. Spoon some of the cooking broth over the top if desired.