Adjust an oven rack to the top position and preheat the oven to 375 degrees F. Line a rimmed baking sheet with foil.
Trim the ends of the hero rolls by about 1-inch. Pulse the ends in a food processor to make soft bread crumbs. Soak 3/4 cup of the bread crumbs in the milk in a large bowl (save any extra crumbs for another use), about 5 minutes. Gently mix the pork with the soaked bread crumbs, parsley, 2 teaspoons mustard, five-spice, 1 teaspoon salt and a few grinds of pepper. Form into 16 meatballs, each about 1 1/2--inches in diameter.
Arrange the meatballs at least 1-inch apart on the prepared baking sheet. Bake until fully cooked through, about 20 minutes. Turn the oven to broil and cook until the meatballs are nicely browned, 3 to 5 minutes.
Combine the barbecue sauce, cherry preserves, jalapeños, jalapeño brine, the remaining 1 teaspoon mustard and 1/2 cup water in a large skillet over medium heat. Simmer until slightly thickened, about 10 minutes. Add the cooked meatballs to the sauce and toss to coat.
Split the hero rolls. Put some meatballs in each, drizzle with extra sauce, and top with baby arugula.
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