Heat the butter or oil in a frying pan over a medium heat and cook the shallots with the pancetta if using, for 5 minutes. Add the button mushrooms, thyme, lemon zest and a good sprinkle of salt. Cook for 5 minutes, stirring. Add the sherry or wine and let bubble then add the cream and bring to a low, steady simmer. Simmer for 5 minutes until thickened slightly.
Cook the pasta until al dente then drain.
Combine the pasta and sauce with the lemon juice, basil and coriander and season with freshly ground black pepper.