First, you need to marinate the chicken. Cut the chicken into thin slices and put them in a bowl. Add Peri Peri sauce, mustard paste, oyster sauce, vinegar, oil, black pepper and salt. Mix well and let it marinate for at least 1 to 2 hours.
Next, you need to grill the chicken. Heat a grill pan over high heat and grease it with some oil. Place the chicken slices on the pan and cook for about 4 minutes per side or until golden and cooked through. Transfer to a plate and keep warm.
Then, you need to make the omelette. In a small bowl, whisk the eggs with salt and black pepper. Heat some oil in a frying pan over medium-high heat and pour the egg mixture. Cook for a few minutes, flipping once, until set. Cut into small pieces and set aside.
Now, you need to make the homemade sandwich sauce. In a small bowl, combine mayonnaise, peri peri sauce, mustard paste, BBQ sauce, lemon juice, pickled jalapenos, salt and black pepper. Mix well and taste for seasoning.
Finally, you need to assemble the sandwich. Toast the bread slices lightly and spread some butter on them. On one slice of bread, spread some sandwich sauce and top with grilled chicken, cheese slice, omelette, cucumber, tomato and lettuce. Cover with another slice of bread and repeat with the remaining ingredients. Cut into halves or quarters and enjoy your grilled chicken sandwich!