Add the corned beef with its spice packet to a large Dutch oven. Add the mustard seeds, bay leaf and chili powder. Pour in the beer and cover the corned beef under 2 inches of water. Bring to a boil, then reduce to a simmer, cover with a lid, turn heat to low and cook for 3 hours.
Add the carrots, rutabagas, turnips and onions after 3 hours, and cook uncovered until the vegetables are tender, another 30 minutes.
Remove the corned beef to a cutting board and slice into 1/4-inch-thick pieces against the grain of the meat. Place on a platter with the vegetables and some Dijon mustard for serving.