For the sauce, wash lemon hot, grate dry, grate peel thinly, halve fruit and squeeze juice from one half. Peel garlic and chop roughly. Put egg yolk, mustard, oil, 2 tbsp. lemon juice and zest in a narrow, tall cup.
Whip into a mayonnaise with a hand blender. Stir in approx. 4 tbsp. water, season with salt and pepper.
Wash the watercress, shake dry, pluck leaves from the stems. Cut garden cress from the bed. Wash apple, quarter it, cut out core and cut apple into thin slices.
Arrange salmon, apple and cress on plates and drizzle with prepared sauce. Use the rest of the sauce for other purposes.