Mix together mustard paste, vinegar, salt, crushed black pepper and fresh oregano in a bowl and mix well. Add chicken cubes and mix and set aside to marinate.
Heat oil in a non-stick wok, add the marinated chicken and sear well. Add mushrooms and salt, mix well and cook for 5 minutes.
Add white sauce and milk and mix well. Add rice and toss well.
Put chicken rice in an ovenproof glass tiffin, add blanched spinach and mix well.
Spread the breadcrumbs and parmesan cheese powder on top. Keep the tiffin in the preheated oven and bake for 10-15 minutes.
Remove it from the oven and let it cool. Cover the tiffin.