Peel and wash the potatoes and cook in salted water for about 20 minutes. Then drain, steam and press through the potato press. Knead with egg yolk and 20 g butter, leave to cool. Knead in about 150 g flour, salt, pepper and nutmeg. Roll into finger-thick strands on a little flour and cut into pieces approx. 2 cm long. Press in lightly with the fork.
Scald tomatoes, quench, peel and dice. Toss with sage in 40 g of hot butter.
Let the dumplings stand in boiling salted water for about 5 minutes, then drain. Serve with the melted sage tomatoes. Grate the parmesan on top.