Cook the potatoes in boiling salt water for around 25 min.
Season the fish with salt and ground black pepper. Drizzle with lemon juice and place in a oiled steamer insert.
Boil some water in a pot, add the steamer insert, cover with a lid and cook the fish for around 8 min.
Mix together the yogurt, mustard, salad cream and 1 tsp dill. Season with salt, ground black pepper and a dash of lemon juice.
Drain the potatoes, leave to cool a little and cut into bite-sized pieces. Roughly chopped the fish and mix together with the potatoes, dressing and the spinach. Season to taste.
Arrange on plates and serve garnished with the remaining dill.