For the pesto: Bring a large pot of well-salted water to a boil. Drop the rabe into the water, give it a swish, and remove it immediately, saving the water to cook your pasta in later.
Reserve a cup of the rabe. Toss the rest in a food processor and pulse, pulse, pulse until you have a coarse paste. Add the pistachios and Parm and puree until smooth. If it seems dry, drizzle in a little olive oil while the machine is running. Add the mascarpone and pulse until combined; taste for seasoning. It should be slightly bitter, nutty, and creamy at the same time. Reserve.
For the pasta: Coat a large saute pan with olive oil and bring to medium-high heat. Add the sausage, using a spoon to break it up, and cook until brown and crumbly, 8 to 10 minutes.
Bring your broccoli rabe water back to a boil and toss in the pasta, cooking for 1 minute less than the package recommends. Drain the pasta and add it, along with 1/2 cup reserved pasta cooking water, the reserved rabe, and two-thirds of the pesto, to the pan with the sausage. Stir to combine and cook until the water evaporates and the pesto is clinging to the pasta. Remove from the heat, drizzle with some big fat finishing oil, sprinkle with more Parm, and stir vigorously to combine. Divide among bowls and serve immediately.