How to Make It
Lay the pancetta on a baking sheet lined with plastic wrap and freeze until firm. Finely chop the pancetta, then transfer it to a food processor and pulse until minced.
In a large bowl, combine the ground pork, veal and beef with the minced pancetta. In a small bowl, combine 1 teaspoon of salt with the cinnamon, allspice, cloves and nutmeg. Sprinkle the spices over the meat and mix thoroughly but gently with your hands. Pat the meat into 6 burgers and refrigerate.
Light a grill. In a large pot of boiling salted water, cook the broccoli rabe until just tender, about 2 minutes. Drain, lightly squeeze out any excess water and coarsely chop.
Heat the olive oil in a large skillet. Add the garlic and crushed red pepper and cook over moderate heat until fragrant, about 1 minute. Add the broccoli rabe and cook, stirring, until heated through. Season with salt and pepper.
Season the burgers with pepper and grill over a hot fire until just cooked through, about 5 minutes per side. Melt the Fontina on the burgers about 1 minute before they are done. Spoon the broccoli rabe onto the bottom buns, then set the burgers on top. Close the sandwiches and serve at once.