Wash the cucumber, slice lengthwise into thin slices. Peel and finely chop the onion. Wash lime hot, dry, grate peel. Squeeze lime. Heat half the juice and onion in a pot. Mix crème fraiche and cucumber in a bowl.
Season to taste with salt, pepper and 1 tsp. sugar.
Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Wash 4 potatoes, cut in half lengthwise. Using a ball cutter or small knife, remove the inside to a rim about 3 mm wide.
Place the potato halves on an oiled tray with the cut surface facing downwards. Bake in the oven for about 25 minutes. Peel the rest of the potatoes, wash and cut into small pieces. Cover the inside of the potatoes and cook in salted water for about 15 minutes.
Finely dice the bacon. Fry until crisp in a pan without fat. Wash parsley and chop finely. Drain potatoes, add milk and 2 tablespoons of butter, mash to a coarse masher. Stir in bacon and parsley.
Season with salt and nutmeg.
Remove the outer leaves from the lemon grass. Press the stick, cut into thirds, boil up with 150 ml water, 1 teaspoon butter, the rest lime juice and zest. Rinse the fish, cover and steam for about 15 minutes.
Take the potatoes out of the oven and fill with mashed potatoes. Rub gouda over them. Bake under the grill for about 8 minutes. Cut the cress from the beds and fold into the salad. Serve with fish and potatoes.