Start by cleaning the steamed mackerel. Carefully flake the fish apart with your hands or two forks into a large bowl. Check the fish meat carefully and remove all bones. Then rinse the four large romaine lettuce leaves under cold water and pat them dry.
Add the finely chopped small red onion, 2 tablespoons of sugar-free mayonnaise, and the juice of 1/4 lemon to the bowl with fish. Sprinkle a pinch of turmeric over it. Mash and mix everything together firmly with a fork until the ingredients are completely combined and form a cohesive, coarse pâté.
Lay the dried romaine lettuce leaves next to each other on a work surface or directly on plates, so they can serve as boats. Divide the mackerel pâté equally in the center of each lettuce leaf and spread the mixture neatly along the length.
Finally, sprinkle a little Celtic sea salt over the mackerel pâté to enhance the flavors of the dish. Serve the filled lettuce boats immediately.