1. For the mustard aioli: In a bowl, combine the mayonnaise, mustard, lemon zest, horseradish, dry mustard, salt, hot sauce, garlic and lemon juice. Mix well. Cover and refrigerate until ready to use. For the Cubano sandwich: Preheat the oven to 180°C.
2. In a bowl, mix together the paprika, cumin, dry mustard, onion powder, celery salt, kosher salt and brown sugar. Rub the pork tenderloin all over with the spice rub. Transfer to a baking sheet and roast to an internal temperature of 73°C, about 25 minutes.
3. Remove the pork from the oven and let rest for 10 minutes. Slice thin and set aside. Heat a large skillet over medium-low heat. Add the pancetta slices in batches and cook until crispy. Drain on a paper-towel-lined plate. Set aside.
4. To assemble, slice a loaf horizontally. Spread the cut sides with 2 tablespoons mustard aioli, then spread the bottom half with 2 teaspoons relish. Top with 1/2 cup of the shredded gruyere, 5 to 7 slices of the pork tenderloin, 3 slices pancetta and then another 1/4 cup of the gruyere.
5. Heat a griddle pan over medium-high heat until hot. Spread 1 tablespoon of the mayo on the exterior of the bread. Place the sandwich in the pan and press firmly with a panini press or heavy skillet. Heat until the bread has toasted and the cheese is melted, 5 to 7 minutes.
6. Remove from the pan, slice in half and serve. Repeat for the remaining sandwiches.