Remove the corned beef from the bag.  Rinse the meat well under cold water and pat it dry with paper towels.  Save the spice bag and set it aside.

Preheat the oven to 325 degrees.

Place 2 large sheets of aluminum foil on a flat surface in a crisscross pattern.

Lay the corned beef in the middle of the foil.

Rub all sides of the beef with the mustard and horseradish. Place the fat side up in the middle of the foil packet. You may want to gather the foil around the meat to create a lip.

Layer the vegetables around the meat.

Sprinkle the meat and veggies with the spice packet, garlic, salt, and pepper.

Partially roll the foil up around the meat, forming a packet.

Pour the beer around the meat and vegetables.

Continue to roll the foil into a packet with a one-inch opening around the top.

Place the foil packet on a rimmed baking sheet.

Bake in the preheated oven for about 2-hours.

Carefully pull the opening of the foil back to expose about another inch of the meat and vegetables.

Use a spoon to baste the top of the meats and vegetables with the beer and juices in the bottom of the foil packet.

Continue to bake for another hour or until a meat thermometer reaches 165 degrees.

Remove the meat from the packet and allow it to rest for 5 minutes.

Slice into thin slices across the grain of the meat.

Serve with the vegetables and drizzle some accumulated pan juices and beer over the beef.