Cook the pasta in boiling water in a large non-stick saucepan according to pack instructions. Meanwhile, slice the mushrooms and snip the bacon into bite-size pieces with scissors or a sharp knife.
Reserve a few drops of the cooking water in a cup or bowl, then drain the pasta and set aside. Fry the bacon and mushrooms in the same pan until golden, about 5 mins. Keep the heat high so the mushrooms fry in the bacon fat, rather than sweat.
Tip the pasta and reserved water back into the pan and stir over the heat for 1 min. Take the pan off the heat, spoon in the pesto and crème fraîche and most of the basil and stir to combine. Sprinkle with the remaining basil to serve.