Peel and wash the potatoes; cut them in half lengthwise and place them in a pan that’s large enough to contain them all, one beside the other, in a single layer. Add the water and milk so it just covers the potatoes (if 1 glass of each isn’t enough, add a little more) and boil over a moderate heat.
In the meanwhile, in a frying pan with the oil, sauté the clove of garlic “in camicia” (literally “with its shirt on”, that’s to say crushed, without removing the skin), thyme and sage. After a few minutes remove the herbs from the pan and add the fish, cut into small pieces. Cook over an average heat, season with salt and pepper, turning the fish on both sides. Turn up the heat, pour in the wine and let it evaporate, then turn off the heat. Leave to cool down and then, in the same pan, use the tip of a wooden spoon to break the fish up so it gets mixed with the cooking juices. Drain the potatoes of any remaining liquid they were cooked in and let them dry for a moment longer in their pan. Put them through a food mill (or potato masher) and add them to the pan with the fish. Carefully mix together and, after tasting, add a pinch of salt if needed: and now the filling for your tortelli is ready.
Prepare the pasta: Pile the flour up on a pastry board (if you don’t have one, use your kitchen work surface), create a "well” in the top of the pile and break the four eggs into the centre, adding a pinch of salt and a tablespoon of olive oil. Use a fork to beat the eggs and oil delicately, then take the flour from the edges and move it towards the centre, a little at a time, as you continue mixing. Once the dough is quite firm, begin to knead it with your hands so that the eggs absorb all the flour. Knead the dough energetically, using the weight of your whole body, every now and again sprinkling a little flour on the work surface to prevent the dough from sticking. Halfway through this process, wash and dry your hands well because the small pieces of dry dough stuck to them will ruin the sheet of pasta while your roll it out. Continue for 10-15 minutes, until you obtain a smooth, elastic ball.
Take a piece of the dough, roll it out as thin as possible (it should be almost transparent when held against the light) and cut it into strips approximately 14 cm wide. You can use the rolling pin as a “ruler”, laying it on the sheet of pasta. Then make little balls of the potato and fish mixture and place them along the centre of the strip of pasta, leaving 4 cm between them. To cover the filling, fold the strip of pasta over lengthways; press down slightly with your fingers and then, with the side of your hand, begin to shape it while pressing on the pasta so that all the air escapes (this helps prevent the tortelli from opening during cooking). Take a 6 cm diameter glass, dip the rim in flour and use it to cut semicircles around the mounds of filling, creating classic half-moon shaped tortelli, then use a fork to seal the edges together well. Continue until you have used all the pasta dough.
Now prepare the sauce: put the oil in a frying pan with the thyme and sage, the clove of garlic "in camicia” (slightly crushed but with its skin on) and the chopped and whole pistachios. Toast the pistachios over a very low heat for about 5 minutes (take care that the oil doesn’t boil); turn off and remove the thyme, sage and garlic.
Cook the tortelli in a large saucepan with plenty of boiling, salted water. Watch out because the pasta is very thin and they will rise to the surface rapidly. Wait 2 minutes, then very delicately use a perforated spoon to strain them, a few at a time. Lay the tortelli on a serving dish and pour the pistachios and oil over them; shake the dish slightly to distribute the sauce and serve immediately!!!