Season the brisket with salt, pepper and add to the pan. Sear on both sides until brown, about 3 minutes per side. Add to the slow cooker and add remaining brisket ingredients to the slow cooker. Cook on low for 6-7 hours.
For the pickles: Heat vinegar and sugar together until sugar is dissolved and pour over radishes and carrots. Set aside to cool.
For the mayo: Combine the mayonnaise and Sriracha and set aside.
To assemble sliders: Spread mayonnaise (made above) on half of the bun and top with shredded brisket. Add two slices of cucumber, 2 tablespoons of the pickled vegetable, cilantro, mint and scallions to taste. Top with remaining half of bun.