The word "greens" in the title of this dish may sound a little vague. What exactly does that mean, and why can't I be more specific? Honestly, because I see no reason to limit you since any leafy, semi-sturdy vegetable will work really well here. That could mean dandelion greens, kale, escarole, or mustard greens, for example, depending on what's in season and at your market. Tender lettuces, on the other hand, should be avoided.
No matter what you get, the greens can be treated the same way: any thick stems should be trimmed and discarded, and then the leaves can be cut into two-inch ribbons. Added to the pasta during the last two minutes of cooking, they quickly wilt and soften under the heat.
As for the rest of this dish, I start with diced pancetta, browning it and rendering its fat. Then I add shiitake mushrooms, along with some shallot and serrano pepper, and cook them until browned.
I scrape the pancetta mixture out of the pot, then fill it with some vegetable stock or broth, which I bring to a boil and cook the fusilli in it. It should be just enough liquid that it will be mostly absorbed and evaporated by the time the pasta is ready.
Towards the end of the pasta-cooking time, I stir in those greens and cook until wilted, followed by the pancetta-mushroom mixture. The rendered fat from the pancetta should blend with the remaining liquid in the pot to form a creamy, emulsified sauce.
A splash of lemon juice and grating of Parmesan is all that's needed to finish it off.