Assemble all ingredients.
Preheat smoker to 275 degrees.
Trim excess fat off corned beef.
In a mortar and pestal, grind the whole juniper berries into almost a paste like consistency. Add peppercorns and continue to grind.
Mix whole grain mustard into the ground spices.
Coat corned beef with mustard and spice paste on both sides.
Place on smoker grate, and smoke for 3 hours with the lid closed. If using a regular BBQ grill, use an indirect heat method and place the meat on the side without flames or coals.
After 3 hours, place the corned beef in a shallow aluminum pan, add beer and surround with onion. The beer will help draw the excess salt out fo the corned beef and bring in complimentary flavors.
Cover the pan with heavy duty aluminum foil and put the corned beef back in the smoker for another 3 hours.
Remove from the smoker and rest for 15-30 minutes before slicing.
Carve against the grain and serve.