Onlays are often referred to as partial crowns. The existing tooth is used as a base to support the restoration. The onlay serves to strengthen the tooth, restore its shape, and help prevent further damage. Onlays are recommended when the tooth can't support a large restoration reliably.
The tooth is prepared by removing any existing filling and decay and then extending the preparation to allow for material. An impression of the tooth is taken and sent to a dental lab. They create a restoration made from porcelain, gold, or a composite resin. The restoration takes 2-3 weeks to fabricate. A temporary onlay will be placed on the tooth for that time. During your next visit the onlay will be tried in and adjusted. It will then be cemented or bonded. The onlay will restore shape and function to your tooth in a conservative manner.