Whisk together all the ingredients for the slather. Set aside.
Combine all ingredients for the rub and mix well.
Cover brisket in slather and allow to sit,covered,for 24 hours in the fridge.
Remove brisket from liquid and coat in the dry rub,then allow to sit for another hour.
Preheat oven to 140C. Place the brisket in a roasting dish,add a splash of beer and cover with foil. Cook on a wire rack in the oven for 10 hours. (Alternatively,smoke at 120C until internal temperature is 90C,about 12 hours.)
For the salsa,add the onion,garlic and jalapenos to a food processor. Pulse until finely chopped. Add tomatillos,coriander,the juice of one lime and salt. Blend to a smooth salsa with a bit of texture.
To serve,blanch spring onions then place on a lightly oiled grill until nicely coloured and soft. Lightly toast tortillas. Slice or tear brisket into large pieces. Place decent amount of brisket in each tortilla,top with a grilled spring onion and finish with a generous spoon of salsa verde and a few coriander leaves.