Preheat oven to 120°C. Use your fingers to pull the fish flesh apart and loosen it up into a shredded mixture. Spread it over four slices of the bread and lay the sliced onion and tomatoes on top. Sit the remaining bread firmly on top (as if you’re making a sandwich) and press down for a few seconds each to help hold it together. Whisk the eggs with the cream and a pinch of salt, then one by one dip the sandwiches in it to coat them on both sides, removing them onto a tray as you go. Pour any excess egg mixture over them as they need to be quite eggy to be really delicious. Heat a pan up (ideally one large enough to take at least two sandwiches at once) and when it’s moderately hot add either the butter or oil. Once that’s hot and sizzling add two sandwiches and cook them over a moderate heat until the eggy bread has turned dark golden. Gently flip them over and cook on the other side until dark golden, then remove and place on an oven tray and put in the oven to keep warm. Cook the next two sandwiches the same way and place these in the oven and cook for 3-4 minutes to ensure they’re warmed all the way through.
Once they’re in the oven make the dressing by mixing the horseradish with the lemon juice and crème fraîche.