A ring lock that accepts a chain or cable is one way to go. The ring lock is permanently mounted on the bike and the chain or cable (made specifically for that lock) is always connected to the ring lock.

My bike cost more than €4000 and I just use a ring lock (for the insurance) and an additional Abus U lock (which is mounted on the bike). That U lock gets used if I'm concerned about the area where I'm parked. Normally though I don't use it. As long as I can provide my insurance company two ring lock keys, I covered for full replacement in case of theft.