Press the potatoes through a potato press. Knead with the egg yolk, flour, nutmeg, salt and pepper to form a smooth dough. Shape the dough into noodles the size of your little finger, tapering at both ends. Cook the pasta in boiling salted water for 4-5 minutes. Then lift out of the water, drain well and allow to evaporate.
Melt the butter in a deep pan and sauté the onion until translucent. Add the pasta and toast until golden brown.
Halve the washed tomatoes. Mix the spinach, garlic, tomatoes and pine nuts into the pasta and sauté briefly. Arrange on preheated plates and sprinkle with the parmesan shavings.