For the spice rub: Preheat the oven to 220 degrees F.
Stir together the 5-spice powder, salt and pepper in a small bowl.
For the brisket braise: Rub spice rub on brisket. Sear brisket in oil in a skillet until brown, then set aside. Line the bottom of a large braiser or Dutch oven with lemongrass, onions, Thai chilies, jalapenos, brown sugar and cilantro. Place a rack over vegetables, then place brisket on the rack. Fill two-thirds the height of the brisket with prepared pho broth. Place the coriander seeds, cardamom pods, star anise and cinnamon stick into a piece of cheesecloth and tie into a sachet. Add spice sachet to pot, cover and transfer to the oven to braise, 12 hours. Remove meat from the braise and chill until cold, about 5 hours.
Cut into 1/2-inch slices and reserve.
For the cucumbers: Meanwhile, combine cucumber slices, sugar, salt and Chile sambal to taste in a bowl. Set aside for 1 hour, then refrigerate.
For the carrot pickle: Boil vinegar, then add sugar, salt and Thai chiles. Pour over carrots in a bowl and set aside to cool, then refrigerate.
For the spicy mayo: Mix mayo, salt, pepper, 5-spice powder and sriracha to taste in a bowl.
For the sandwich: Butter the cut sides of the hoagie rolls and grill them until toasted. Coat each side with spicy mayo. Sear off 3 or 4 slices per sandwich of cooled brisket until hot, then layer them on roll bottoms, followed by spicy carrot pickle, pickled cucumber, fresh Thai Basil, cilantro, mint and white onion slices. Close sandwiches and cut in half on bias. Plate with bean sprouts, lime wedges and jalapenos. On side, fill a bowl with the pho soup broth, chile oil, cilantro and green onions for dipping, as one would a French Dip Sandwich.
Have lots of friends over; it's a big piece of meat!
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.