a meat grinder with stuffing attachment; teasing needle or sausage pricker
Preheat the oven to 400 degrees F. Chill a large metal bowl and all the grinder parts until very cold.
For the sausage: Assemble the grinder with the chilled parts using the large-holed grinding plate. Combine the pork, garlic, fennel pollen, salt, crushed red pepper flakes, fennel seeds and Parmesan in the chilled metal bowl. Pass the meat through the grinder back into the chilled metal bowl. Switch to the medium-size plate and pass the ground meat through again. Add 1/4 cup ice water and quickly knead the meat mixture together with your hands.
Fit the horn (stuffing attachment) onto the grinder. Grease the horn with oil and gently slide the hog casing onto the attachment, leaving about 2 inches at the end. Add meat to the hopper and turn the grinder on. Let the air run through the casing until the meat starts to come out. Once you see the meat, turn the machine off and knot the casing end. Turn the machine back on and keep feeding the meat mixture into the machine. Using your other hand, guide the casing off the horn as it fills. Continue until you run out of casing, leaving a short tail on the end.
Form the filled casing into links by gently pinching and then twisting it. The sausage links should be equal in size, 3 to 4 inches each. Tie each twist with twine and puncture the links all over with a teasing needle or sausage pricker. You should have 8 to 10 links total.
For the toppings and assembly: Bring a large pot of lightly salted water to a boil. Blanch the broccoli rabe until bright green, about 30 seconds then use a slotted spoon or spider to transfer to a bowl. Leave the water on the heat for the sausages. When cool enough to handle, Squeeze out the excess moisture in the broccoli rabe and roughly chop. Set aside.
Reduce the blanching water to low heat so it is hot, but not bubbling. Carefully drop the sausages into the hot water and poach for 10 to 12 minutes, flipping halfway through, or until they reach an internal temperature of 150 to 156 degrees F. Remove from the poaching liquid and place on a baking sheet.
Heat 2 tablespoons olive oil in a medium saute pan and add the onions with a pinch of salt. Sweat the onions until translucent, 5 to 7 minutes, then add the cherry peppers and broccoli rabe. Toss everything together and add a splash of the pickled pepper brine. Season to taste with salt. Remove to a bowl and wipe out the saute pan.
Heat 1 tablespoon of olive oil in the same pan over medium heat and brown sausages on both sides, 3 to 4 minutes per side. Lower the heat if they start to burst.
Open the hoagie rolls and use your fingers to scoop out some of the bread from the top part of the roll. Brush the cut sides of each roll with olive oil and place open faced on a baking sheet. Toast in the oven until warmed through until lightly golden, about 5 minutes.
Lay provolone slices on the toasted rolls and melt in the oven, 3 to 4 minutes. Cut the sausage links apart, remove the strings and slice on a bias. Lay the slices over the melted provolone and top with the broccoli rabe-pepper mix and more provolone. Transfer back to the oven to heat through and continue melting cheese, 3 to 5 minutes.