Word choice for salutation in a formal email to your PhD adviser, Is *Dear Boss* appropriate?
In my view, in academia, the word "Boss" carries a large amount of semantic baggage that you probably don't want to invoke. Although you mean it to be polite and respectful, it also has an ironic context which you may not have detected. By saying "Dear Boss" can also imply that in your opinion they are neither dear to you or deserve the respect as a superior. It is safest to avoid such salutations altogether in emails.

As others have indicated this is very culturally dependent and I think you are bringing too much of your south asian culture to a european environment. Do what the locals do and emulate other peers is the safest and wisest approach.