There are the open sandwich recipes that never make it to the books but that are so easy, so traditional.... This one, my mother served for us for lunch as kids. In fact. most Danish mothers will have served this to their kids. It's one of those open sandwiches we grow up on. It's unlikely you'll ever find it in any book, but that doesn't make it any less delicious.
Dark seeded rye bread topped with mackerel and tomato straight from the tin. A dollop of (good quality) mayonnaise - and season. It's really delicious and is ready in about 24 seconds.