Add all of the ingredients to a large (6-7 quart) slow cooker, and toss to combine. (I use low-sodium chicken broth with about 1/2 teaspoon each kosher salt and ground black pepper. You may wish to add half that amount and adjust seasonings later if your broth is not low in salt.) Cover and cook on low for 6-8 hours, or until the beef and vegetables are tender. (I think the corned beef stays more tender when cooked on low, but if time is short you can cook on high for about 3 hours.)
Transfer the corned beef chunks to a plate, cool slightly, and then dice. (I prefer fairly small pieces.) Return the beef to the slow cooker, and stir to combine. Taste and season with additional salt and pepper if needed, then remove the bay leaf and/or thyme sprigs.
Serve warm, garnished with fresh herbs if desired.