Preheat your oven to 325°F (163°C).

Rinse and dry the corned beef brisket gently with paper towels to remove any excess brine.

In a mixing bowl, combine whole mustard seeds, coriander seeds, peppercorns, minced garlic, brown sugar, paprika, crushed red pepper flakes (if using), and thyme to make the spice rub.

Brush the brisket generously with olive oil or mustard and sprinkle the spice mixture evenly over the brisket.

Place the seasoned brisket in a roasting pan and pour beef broth (or water) into the bottom, covering about ½ inch.

Roast the brisket for about 2 ½ to 3 hours until the internal temperature reaches 190°F (88°C).

Once done, remove from the oven and let it rest loosely covered with foil for 15 minutes.

Slice the brisket thinly against the grain, serving warm alongside zesty mustard.