1To make the spiced barbecue rub, put the paprika, dried onion, dried garlic, peppercorns, mustard seeds, salt and cayenne in the blender and secure the lid. PUREE for 45 seconds until the peppercorns are finely chopped. Add the brown sugar and secure the lid. BLEND for 15 seconds. Transfer ½ cup (2 ¾ oz) of the rub to a bowl for the homemade spicy barbecue sauce and the remaining ¼ cup (1 ½ oz) spice mixture to another bowl to rub over the pork.
2To make the homemade spicy barbecue sauce, put the ingredients in the blender and secure the lid. PUREE for 40-50 seconds until smooth. Pour the sauce into a large saucepan. Bring to a boil over medium heat, stirring occasionally. Reduce the heat to low, cover and simmer, stirring occasionally, for 30 minutes until the sauce has thickened. Season with salt and pepper. Set aside to cool. Transfer to the blender and secure the lid. BLEND for 10 seconds until smooth. You will need 1 cup (8 fl oz) for this recipe. Transfer the remaining sauce to a clean jar and store in the refrigerator for up to 2 months. Use as a marinade over barbecue ribs, steak or chicken. Use as a sauce on pizza, burgers or wraps.
3Rub the reserved ¼ cup (1 ½ oz) spiced barbecue rub all over the pork. Put in a resealable bag. Seal and refrigerate for 4 hours, or overnight.
4Remove the pork from the refrigerator and let stand for 30 minutes to bring back to room temperature.
5Preheat oven to 300°F.
6Heat the oil in a large Dutch oven or similar, heavy-duty pot over medium-high heat. Add the pork and cook, turning occasionally, for 10 minutes until browned all over. Add enough of the stock, to come halfway up to the pork. Press a piece of parchment paper onto the surface of the meat, then cover with a tight-fitting lid. Transfer to the oven and bake for 4 hours until the pork is fork-tender. (Alternatively cook in a slow cooker for 4-5 hours on High or for 8-10 hours on Low.)
7Transfer the pork to a baking sheet. Using 2 forks, shred the pork, discarding any fat and gristle. Strain the pan juices from the pork into a large measuring cup. Pour 1 cup (8 fl oz) into a small saucepan. Add ¾ cup (6 fl oz) of the barbecue sauce. Stir over medium heat until hot. Pour over the shredded pork and, stir gently to coat.
8Spread the buns with mayonnaise, top with pulled pork, dill pickles, jalapeno, spinach and a dollop of the remaining barbecue sauce. Sandwich with bun tops.