Heat oven to 425 degrees F. Butter a 9-inch springform pan and wrap the outside tightly in aluminum foil. Place on a baking sheet.
Chop pancetta into 1/2-inch cubes and saute over medium heat until well browned. Remove with slotted spoon and drain on paper towels.
Bring a large pot of well-salted water to a boil. Boil the broccoli rabe for 1 to 2 minutes in the salted water until it has some give. Fish it out with a large slotted spoon and drain. Set aside.
Now add the spaghetti to your boiling water and cook until 2 minutes shy of done, so very al dente. While the pasta cooks, use a towel to squeeze all excess moisture from the broccoli rabe. Separate out 1 cup of broccoli rabe and chop it finely. (Reserve the remainder for another use.)
Drain the pasta well and let cool slightly.
In large bowl, whisk eggs, yolks, milk, salt, pepper and all but 1/2 cup of cheese together. Add the spaghetti, chopped broccoli rabe and pancetta and toss to combine. Pour mixture into the prepared springform. Sprinkle last 1/2 cup cheese on top.
Bake 35 to 40 minutes, until bronzed and the center is set. The best way to check is to tip your pan to the side and make sure no loose custard runs out. If it does, bake it for 5 minutes more. If the top of your pie browns too quickly before the center is set, cover it with foil for the remaining cooking time.
Remove from the oven and let cool on a rack for 5 minutes before removing the foil, cutting around springform to loosen and releasing the pie onto a serving plate. Serve in wedges.