Onlays and restorations, which replace part of a damaged tooth. Differently from a crown, these restorations are bonded to the remaining tooth structure and there is no need to prepare the tooth all around, preserving the tooth structure to the maximum.
When the external (vestibular) surface of the posterior teeth is also affected, another very conservative treatment could be choosing over a crown : a veneer/onlay.
A veneer/onlay has a "L" Shape, since one part will cover the masticator (occlusal) surface of the tooth and the other external one. It is a very demanding restoration but when well executed, it offers an extremely ethetic and conservative outcome.