Heat oil in a pan, add onion and fry it till it's soft.
Add ginger garlic paste and chicken in the pan. Cook it for 2 minutes.
Now add 1/2 tbsp salt, black pepper, green chilies, Hp Sauce, ketchup, chili garlic sauce in the chicken and cook it until the chicken is tender.
Once the chicken is tender, turn off the heat and keep the chicken aside.
For White Sauce:
Now take another pan, add butter, flour, chicken stock, white pepper, 1/2 tbsp salt and mustard. Mix them together through a whisk. Keep whisking it to avoid formation of lumps. Once the sauce is thick take it off the heat.
Add the white sauce in the chicken and mix them together.
Dish out the chicken in a baking dish. Arrange the croutons on top of it. Then sprinkle cheese and parsley.
Put the dish in pre-heated oven on 180 degrees for 10 minutes.
After 10 minutes take it out of the oven and server hot.