Wash and remove any impurities from the dandelion leaves, then scald them in water for a few minutes and drain. Cut up the onion, brown it in a little oil and butter and add the chopped dandelion leaves. Cook for a few minutes and adjust the flavor. Allow to cool for a few minutes and add 200 gr. of the ricotta cheese together with some grated parmesan cheese. Mix the flour together with 12 egg yolks, water and a little oil, just enough to make the resulting dough soft and elastic. Wrap the dough in cling film and let it rest for 15 minutes. Roll out the dough fairly thinly and cut into squares of about 6 cm wide. Put the filling in the center and form them into ravioli. In a non-stick frying pan fry the remaining ricotta cheese until it is golden in color. In a small-based saucepan with high sides, melt the butter, add a little cold water and shake the pan so as to froth up the butter slightly. Cook the tortelli in boiling salted water, then serve them up with the browned ricotta, the frothed butter and garnished with some dandelion leaves.