Preheat the oven to 180 degrees Celsius.
Line a roasting pan with baking paper, arrange the chicken, drizzle with oil, sprinkle with seasonings, and rub thoroughly.
Bake for 20-21 minutes.
Once slightly cooled shred the chicken.
Heat 1 tablespoon of oil in a pan over high heat, add the chopped onion and pepper, and cook for 2-3 minutes until softened.
Stir in the tomato puree and seasonings, then add the chopped tomatoes and water; mix well and simmer for 3-4 minutes.
Add the shredded chicken and mix thoroughly.
In a small saucepan, whisk together the milk, cayenne pepper, and flour, then stir in the grated four-cheese blend and simmer for 1-2 minutes.
Arrange the bottom bun halves in a baking dish, layer with mozzarella slices, the chicken mixture, finely sliced spring onions, nacho sauce, and finally, the finely chopped pickled jalapeños.
Top with the remaining bun halves.
In a small bowl, combine melted butter, chopped parsley, garlic granules, and cayenne pepper; brush this mixture over the assembled buns.
Bake for 12-14 minutes at 180 degrees Celsius.
