To make the corned beef:

1. Toast the peppercorns and mustard seeds over medium heat in a dry skillet until fragrant, about 3 min. If you hear the mustard seeds popping, it’s time to take the skillet off the heat.
2. Scrape into a small food processor with the rest of the pickling spices and blitz until ground into a powder. You could also use a mortar and pestle or a blender.
3. Place the water, salts, sugar, garlic cloves and half of the pickling spice you just made into a pot that’s roughly the same size as the brisket.  Bring to a boil over medium heat, stirring from time to time. Lower the heat and let it simmer until the salts and sugar have completely dissolved.
4. Take the pot off the heat and let the brine cool down to room temperature. Once cooled, put the brisket in the pot and weight it down with a small plate to make sure it’ll stay entirely covered.
5. Place the pot in the refrigerator and keep it there for 5 to 10 days. Flip the brisket every other day. Then remove the brisket from the brine (discard the brine) and rinse it well under cold water.
6. Wash the pot thoroughly and put the brisket back inside. Add the rest of the pickling spices and enough water to cover the brisket amply. Bring to a boil over high heat then lower the heat, cover the pot and gently simmer for 4h or until the corned beef is fork-tender. Make sure that there is always enough water in the pot to cover the meat.
7. Remove the corned beef from the pot and discard the cooking liquid. Slice!

To make the corned beef sandwiches:

1. Preheat the oven to 200°C (390°F) and line a baking sheet with parchment paper. Split the buns in half and place them, cut side up on the sheet.
2. Top the bottom halves of the buns with an equal amount of corned beef. Place the sliced cheese on top.
3. Bake for 6 min or until the cheese is completely melted and the buns are golden. Remove from the oven, add the pickles and spread the onion chutney on the top buns. Place the top buns on top of the sandwiches and enjoy.