Turn the stove heat on medium and heat the skillet. Add butter and olive oil.
Add pancetta and sage leaves. Cook until the pancetta starts to wrinkle and the fat part starts to get translucent and a bit golden. Turn it on the other side and cook for another minute or until it is nice and crisp, the fat is golden but not brown. Cook until the sage leaves start to crisp up but not brown.
Take out of the skillet and set on the kitchen towel. (Note 5)
Turn the heat on medium to high and add the mushrooms and cook for a couple of minutes.
Add garlic and cook for another 30 seconds.
Add the leeks and turn the heat down to medium. Saute leeks and mushrooms for another half a minute then add the cream and gnocchi water.
Add salt and freshly ground black pepper to your taste. Let the sauce boil for half a minute or until it thickens a bit. This should not be too long.
Turn the heaf off and add the gnocchi. Mix slowly with the spatula couple of times, just to coat the gnocchi. (Note 6)
Add the parmesan and slowly mix so that all the gnocchi are nicely coated with the sauce. (Note 7)