To make the potato gnocchi, chop the potatoes into even-sized pieces and cook them in a pan of boiling water until tender. Meanwhile, make the tomato and basil sauce. Heat 1 tablespoon of olive oil in a pan and fry the onion for 3 minute3 to soften. Add in the tomato puree and cook for a further minute. Pour in the tomato pup, water, garlic flavour pot, dried oregano and season well with salt and pepper. Mix the sauce and turn down the heat. Cover the pan with the lid and gently simmer the sauce for 10-15 minutes. When the sauce is ready, stir in the chopped basil leaves and turn off the heat. When the potatoes are soft and cooked, 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, light 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 in 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. 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 in a little olive oil. Serve the potato and Parmesan gnocchi drizzled with the tomato and basil sauce and sprinkled with extra grated Parmesan cheese.