1. Preheat oven to 250 degrees Fahrenheit (120 degrees Celsius).
2. Place pastrami roast into a 9×13-inch (23×33-centimeter) pan filled with about two to three inches (five to seven and a half centimeters) of water. Cover tightly and bake for four to five hours or until extremely tender. Drain the water and shred the pastrami using two forks to pull the meat in until fully shredded.
3. While pastrami is baking, heat two tablespoons oil in a large frying pan. Add diced onions and sauté over low heat until soft, brown, and caramelized, at least 35–45 minutes. Add brown sugar, ketchup, barbecue sauce, mustard, and vinegar and mix well to combine. Add to shredded pastrami and mix well until combined.
4. Heat eggplant cutlets according to package instructions, until crisp. To serve, mound about two to three tablespoons of pulled pastrami mixture onto each warm eggplant cutlet. Drizzle with spicy mayo and top with jalapeño peppers and radishes, if desired.