Makes about 1 cup
Bring a large pot of water to a boil and add 2 tablespoons kosher salt. Add the broccoli rabe and cook until tender, about 7 minutes. Drain and transfer to a bowl of ice water to stop the cooking; drain well.
With a food processor running, drop in the garlic and finely chop it. Add the broccoli rabe and pine nuts and pulse until finely chopped. Add the mustard and blend well. With the food processor running, drizzle in the oil. Transfer to a small bowl and stir in the Parmigiano.
The pesto can be stored in a tightly sealed jar, topped with a thin layer of extra virgin olive oil, for up to 1 week in the refrigerator.