Whisk the pineapple juice, soy sauce, sesame oil, brown sugar, garlic, and black pepper in a baking dish until the sugar dissolves. Put the meat in a large zip-top bag, pour the marinade over the meat, seal the bag and knead a bit with your hands to work the marinade into the beefl Refrigerate overnight.
Preheat the broiler on low.
Heat 1/4 c of the olive oil in a large skillet over medium-high heat. Add the onion, bean sprouts, mushrooms, spinach and sa;t and pepper to taste and cook, tossing, until the mushrooms are just cooked through and spinach wilts.
Heat another skillet over medium-high heat. Heat the remaining 1/4 c olive oil. Add the meat, in a batches if necessary, to make a single layer; cook, pressing with a spatula and stirring occasionally, until the meat reaches the desired degree of donenss; medium-rare is best.
Slice the rolls, leaving them attached on one side. Put them open-faced on a baking sheet and top with some the meat. Add layer of sauteed begetables and top with some mozzarella. Broil just until the cheese melts, and serve hot.