1. In a large bowl, toss cauliflower with buttermilk; set aside for 15 minutes.
2. Mix cornstarch, flour, and Cajun seasoning in a shallow bowl. Dredge cauliflower, shaking off excess.
3. Fry in 350°F oil until golden and crisp, 3–4 minutes. Drain on paper towels.
4. Toss hot fried cauliflower in warmed Al Pastor Sauce.
5. Mix mayonnaise and hot sauce; spread on both sides of toasted rolls.
6. Build with lettuce, tomato slices, sauced cauliflower, and top roll.
Hold cauliflower warm in a perforated pan over a steam table before tossing in the sauce to keep its crispness.