In a large pot or Dutch oven, add brown sugar, curing salt, kosher salt, allspice berries, cloves, ginger, mustard seeds, peppercorns, along with the water.
Cook on high just until the salt and sugar dissolve, remove from heat. Add 2 bottles of stout (reserve the last bottle for cooking) and ice, stir until ice has melted and brine is cool.
Add the brisket, cover with a lid and refrigerate for 5 days and up to 10.
Remove from brine and rinse well. Discard the brine and clean the Dutch oven.
Place the brisket back in the cleaned pot, along with the onion, pour the remaining bottle of stout and then cover with cold water until the brisket is fully cover with about one inch of water above the beef.
Bring to a low boil, cover and reduce heat to maintain a gentle simmer. Simmer for 3 hours or until the meat is fork-tender. Move to a carving board, thinly slice against the grain.
While the brisket cooks, make the pickled slaw. In a saucepan over medium heat, add the lemon juice, vinegar, salt, sugar, ginger, allspice, cloves and peppercorns in a pot.
Bring to simmer just until the salt and sugar dissolve, remove from heat. Allow to cool to room temperature.
In a large bowl, add the cabbage and onion. Pour the cooled pickling liquid over the cabbage and onion, refrigerate for one hour and up to a week (this can be done well in advance).
Fill the tortillas with beef and top with pickled slaw.