This seared tuna starter with pickled red onions, caper berries and honey mustard mayo is inspired by the most amazing appetizer I had in an Italian restaurant in Tivoli in Copenhagen about 15 years ago. Sadly, the restaurant is no more, but all these years I’ve never forgotten that appetizer! For many years I didn’t dare try and copy it for fear of being disappointed, but finally I decided to give it a go, and the result here is so good. I really should have done this years ago! When I was a kid we had lots of tuna, but back then in the ’80s and ’90s the tuna would…