Crush garlic, 1 tbsp peppercorns, allspice and ½ tsp coriander seeds in a mortar and pestle until a coarse paste forms, transfer to a large saucepan with salt, sugar, vinegar, ½ tsp yellow mustard seeds, cayenne pepper, ground ginger and 2½ litres water. Bring to the boil over medium heat, stirring to dissolve sugar. Remove from heat, cool completely.
Place brisket in a large non-reactive container, pour over liquid, cover and refrigerate for 3 to 5 days to cure.
Preheat oven to 130-140C. Remove brisket from liquid (discard liquid) and place in a large casserole. Cover completely with water, bring to the boil over medium heat, cover with baking paper and lid, cook in oven until tender (3-4 hours). Cool brisket in liquid, refrigerate until completely chilled (3-4 hours).
Drain brisket (discard liquid), pat dry with absorbent paper, brush with molasses. Coarsely grind remaining peppercorns, coriander seeds and yellow mustard seeds in a mortar and pestle, then press onto all sides of brisket and refrigerate, covered, until required. Pastrami will keep refrigerated in an airtight container for 1 week.
For coleslaw, place cabbage, carrot and onion in separate non-reactive bowls. Bring vinegar, sugar and 120ml water to the boil in a saucepan, stirring to dissolve sugar. Divide among bowls, refrigerate until chilled (15-20 minutes). Drain well (discard liquid), combine in a bowl with oil, season to taste, set aside.
To serve, spread half the bread slices with mustard, layer with thinly sliced pastrami and coleslaw, season to taste, top with remaining bread slices and serve immediately.
Note Tellicherry peppercorns are high-quality whole black
peppercorns. You can substitute another type. Wagyu brisket is
available from specialist butchers. You will need to order ahead.
You can substitute regular brisket.