Cover corned beef brisket with mustard and brown sugar. Place in a large Crock Pot or slow cooker. Arrange carrots, parsnips, baby onions and potatoes around brisket and cover with the Guinness Stout.
Heat everything, covered, on low for 8-10 hours. During the 9th or last hour of cooking, add the cabbage wedges and allow to soften to your desired texture.
If using Guinness, give it a taste with a spoon and add a bit more brown sugar if it seems bitter. But the flavors will mellow.
When ready to serve, remove corned beef and let sit on a cutting board about 10 minutes before slicing. Then, cut into slices against the grain. Serve in a bowl with the cabbage, veggies and a bit of broth.
* Important: If using chicken broth instead of beer, omit the sugar since it’s meant to counteract the bitterness of the Guinness Stout.