If I take the contents of a finasteride pill, mix water, oil or onion juice with it, and apply it to my hair, will this work?

Grinding up a pill and mixing it with water, oil or juice will not make a safe, effective topical treatment.  I'd suggest a consultation with a local hair restoration physician to discuss other options such as minoxidil (Rogaine), laser (LLLT) and platelet-rich plasma (PRP), as well as to get accurate information about oral finasteride.  

You can not grind up the pill of finasteride and mix it with anything that you think of and expect it to work on the scalp. There are topical forms of finasteride which you can purchase