Cut the soup greens into cubes and distribute them in a baking dish.
For the rub, mix roughly half a bowl of cane sugar with roughly a quarter of a bowl of gyros spice. Optionally add a pinch of cajun spice.
Season both sides of the brisket with salt, apply 2 to 4 tablespoons of mustard per side, and spread the spice mixture on each side. Place the brisket on top of the vegetables, put the dish in 2 bags, and refrigerate for 24 to 48 hours.
Preheat the oven to 110-120°C (230-250°F) and place the brisket inside. Cook for 10 to 12 hours, preparing the broth from scratch if making it yourself.
After 7 hours of cooking, pour the accumulated liquid from the baking dish into the broth and reduce it. After another hour, optionally add some of the reduced sauce to the brisket to keep the underside moist.
The brisket is done when it reaches a core temperature of approximately 94°C (201°F) or after approximately 11 hours of cooking.