Tip both yoghurts, the lemon juice, 400ml water and half a teaspoon of salt into the pan (there’s no need to wipe it down first) and put on a medium heat. Warm through for three to four minutes, until the mixture just starts to steam (don’t cook it for any longer, or it will split). Add the pasta, toss to coat and leave to cook slowly for six to seven minutes, until the pasta is cooked and the sauce has thickened.