Peel and finely dice the shallot. Finely dice the cucumber. Chop capers finely. Pluck parsley leaves from the stalks, wash, dab dry and chop finely. Chop anchovy finely. Mix shallot, cucumber, capers, parsley, anchovy and mustard.
Beat the oil drop by drop into the egg yolks until the mixture becomes stiff. Stir in yoghurt. Stir the mixed ingredients into the mayonnaise and set aside. Whisk the egg and season with salt and pepper.
Meanwhile cook the spaetzle in boiling salted water for 2-3 minutes. Drain. Melt butter in the pot. Swivel the spaetzle in it.
Serve the mushrooms with the tartar sauce. Garnish with parsley.