Start by peeling the three hard-boiled eggs. Place them in a bowl and mash them roughly with a fork, retaining some texture. Next, dice the half gherkin into very fine pieces and chop the chives into small pieces. Rinse the four large Romaine lettuce leaves under running water and pat them gently dry with kitchen paper.
Add the finely diced gherkin and chives to the bowl with the mashed eggs. Stir in the sugar-free mayonnaise and mustard. Mix everything well with a spoon until the ingredients are evenly distributed and a cohesive egg salad forms.
Lay the dry Romaine lettuce leaves like boats side by side on a plate or large board. Divide the freshly made egg salad into four equal portions. Neatly scoop the salad into the hollow of each lettuce leaf.
Sprinkle the garden cress in an even layer over the filled lettuce leaves. Finish the dish by grinding some fresh black pepper over it to taste and serve the egg salad immediately.