Heat a saucepan with salted water and cook the tagliatelle al dente.
While the noodles are cooking, heat a small pan with butter and sauté the onions and ham in it. Deglaze with the white wine, simmer briefly and add the bouillon. Squeeze the garlic and let everything simmer for 2-3 minutes. Mix the mascarpone and egg yolks together and carefully stir into the boiling sauce. Heat the sauce well again, season with a little salt.
Pour off the hot tagliatelle and put back in the pan, immediately add the sauce to the tagliatelle, add half of the chives and mix everything well until it sets a little.
Arrange the tagliatelle on preheated plates with a little parmesan and chives and season well with the pepper mill.