Add mayonnaise, onion, mustard paste, lemon juice, salt and black pepper in a bowl and mix well, the sauce is ready.
Now boil Kashmiri red chillies in vinegar.
Then add red chillies, garlic cloves, coriander, mustard paste, vinegar, olive oil, stearic acid, lemon juice and soaked Kashmiri red chillies in a blender and blend well.
To make finger fish, add salt, chili powder, chili powder, peri peri sauce, egg, flour and mustard paste in a bowl and mix well to make a paste.
Now put fingerfish in it and coat it and keep it for marinating for 30 minutes.