I agree with the other forum members in removing the whole tooth ,I noticed when I started having my amalgam fillings removed that just having a few of the fillings removed I seen an improvement in my health just with the small amount having been removed , even the best dentists can make mistakes and can release small amounts of mercury back in your system from drilling and the dental material used for the onlay may effect you in a negetive way also , and as so many of us here have found out can effect you for you for a while .if it were me ,I would not take that chance with so much riding on it.( when in dought ,pull it out)