Blanch the sweet potatoes in sufficient salted water for 4-5 minutes. Transfer into a bowl of ice cold water; keep for some time and drain. Pat dry on a muslin cloth.
Heat sufficient oil in a pan. Deep-fry the strips till golden and crisp. Drain on absorbent paper.
Sprinkle salt on them and set aside.
Preheat oven to 180° C.
Heat butter in a non-stick pan. Add refined flour, mix and sauté for a minute.
Add milk and cook till the mixture thickens stirring continuously. Add salt, crushed peppercorns, ½ teaspoon chili flakes and mixed herbs and mix well.
Add processed cheese and mix till it melts.
Transfer the sweet potato fries into a casserole. Pour the cheese sauce and top with mozzarella cheese. Sprinkle remaining chili flakes and crushed peppercorns, place the casserole in the preheated oven and let the cheese gratinate.