Cut the fish into roughly ¼-inch cubes. Then place it in the fridge while you prepare the remaining ingredients.
In a medium mixing bowl, add the cucumbers, crème fraîche, horseradish, cornichons, capers, lemon juice, parsley, chives, dill, black pepper, and flaky sea salt. Give it all a nice stir to combine and then fold in the mackerel, making sure every piece is well coated.
Spoon it onto a serving plate in a single layer, then drizzle the olive oil over the top.
Crack a few more twists of black pepper over the top, then sprinkle it with additional parsley, chives, and dill to make it look fun and vibrant. Serve immediately, with crackers.