Boil the stock with the rosemary until reduced by half. Then whisk in the crème fraiche and boil again for 3 mins. Strain the sauce then return to the pan.
Cook pasta until al dente.
If using fresh beans, pod them and boil for 2 mins in salted water until tender. Drain. There’s no need to boil frozen beans, just thaw and pat dry.
Heat the oil in a large frying pan. Stir fry the bacon for about 3 mins then toss in the beans and heat for about 2 mins. Sprinkle with lemon zest and mix into the reduced creamy stock.
Then add pasta, check seasoning, reheat until just boiling and serve sprinkled with the chopped herbs.