For the spice rub: Combine all ingredients except the olive oil in a small food processor, mortal and pestle or spice/coffee grinder. Add the olive oil and process until you have a paste. Set aside.
Prepare the pork: Using a sharp knife and cutting from the long side, cut underneath the fat cap, between the fat and the meat almost, but not all the way across to the other side. Once cut, fold it back out of the way. Cut down the centre of the pork, lengthwise, almost but not all the way through. Fold it open like a book. Repeat this cut with the two pieces you just cut. Press to lay flat out a cutting board. Cover with plastic wrap and using a mallet or heavy skillet, pound down to flatten a bit. Remove plastic. Rub 1/2 of the spice rub on top of the pork. Starting from the end without the fat, start rolling up the pork and when you get to the fat cap, fold it over the top. Rub the outside of the pork with the remaining spice rub. Secure the pork with cooking twine every 2 inches. Refrigerate uncovered at least 2 hour, or preferably over-night.
To cook the porchetta: Preheat oven to 475° F. Place porchetta fat side up in a shallow baking pan and place into preheated oven. Cook for 20 minutes, then reduce heat to 325° F. and cook until thermometer registers about 135°. F in the thickest part, about 75-90 minutes (for 1 1/2 lb. roast). Allow to rest at least 20 minutes, then remove the string, slice and then roughly chop. Place in a bowl and toss with some of the pan drippings. Proceed to make tacos immediately or refrigerate, covered, for up to several days.
Make the Quick Pickled Red Onions: Boil a kettle of water. Pour boiling water over onion rings in a large bowl until covered. Let stand 2 min, then drain. In a medium saucepan on the stovetop, boiL the vinegar with the sugar, salt and garlic until sugar dissolves. Remove from heat and add onion rings. Push onion down into liquid so it is mostly covered and let stand for at least 30 min. To store longer, cover and refrigerate. Can be refrigerated up to 1 week.
Blanch the broccoli rabe: Bring a large pot of water to a boil. Prepare broccoli rabe by removing the bottom couple of inches of the stems. Place broccoli rabe into boiling water and allow to cook 1-2 minutes. Remove to a colander and rinse briefly under cold water to stop the cooking, then set aside to drain. Before using, blot dry between paper towel and re-warm, if desired. Can be made ahead and refrigerated. Use at room temperature or slightly re-warmed.
Prepare the Chipotle Mayo: Combine the mayonnaise and adobo sauce, to taste. Refrigerate if not using immediately.
To assemble the tacos: Place 3 stalks of blanched broccoli rabe in the centre of the tortilla. Top with chopped porchetta. Sprinkle with some of the pickled red onion and scatter some chopped cilantro/parsley on top. Top with a dollop of chipotle mayo and serve with a lime wedge, for drizzling.
You can use either a boneless pork loin or boneless pork shoulder, or if you are lucky enough to have access to a large piece of pork belly, that would be ideal.
All of the components of these tacos can be made ahead and refrigerated. You can serve the broccoli rabe and/or porchetta at room temperature or re-warm, if desired.
*You can substitute chipotle chilli powder for the adobo sauce, or even smoked paprika with a little cayenne, for some heat. Adjust amount of any of these to your taste.
For a different taste, try a homemade salsa verde in place of the chipotle mayo.