To make the potato gnocchi, chop the potatoes into even-sized pieces and cook them in a pan of boiling water until tender. Remove the boiling water from the pan and quickly mash the potatoes. Next, pass the potato mash through a fine sieve using the back of a spoon. This will remove any lumps and give the gnocchi a fine, lighter texture. Add the grated Parmesan cheese and egg yolk to the potatoes and season well with salt and pepper. Mix the potatoes well with a spoon before adding in the flour. Quickly work the flour into the potato to produce a nice, smooth gnocchi dough. Turn the dough out onto a lightly floured work surface and knead lightly for a minute. Cut the dough into three or four pieces and roll each piece into a long, tubular shape, about the thickness of your finger. Cut off pieces 2.5cm/1in long using a sharp knife. You can leave the gnocchi into this shape, or alternatively you can roll each piece into an oblong and press onto the back of a fork to make indentations in the gnocchi. As you go along, place the gnocchi onto a tray sprinkled with the semolina, and also sprinkle some on top of the gnocchi to avoid them from sticking together. You can now prepare the sauce for the gnocchi by heating the olive oil and butter in a large pan. Add in the garlic and thyme and cook for 2 minutes. Add the diced pumpkin to the pan, season well with salt and pepper and continue to cook for about 5 minutes only, leaving the pumpkin still firm. Add the double cream and white wine to the pan, lower the heat and allow the sauce to reduce and thicken for 2-3 minutes. Add in ¾ of the chopped walnuts and turn off the heat. To cook the gnocchi, drop batches into a large saucepan of salted boiling water, taking care not to over crowd the pan as the gnocchi will stick together. As soon as the gnocchi are cooked, they will float to the top of the water (this should take only 2 or 3 minutes). Remove the cooked gnocchi from the pan with a slotted spoon and toss into the prepared sauce. Coat the gnocchi with the sauce and serve immediately in deep plates sprinkled with the remaining chopped walnuts.