SHERRY-MANGO SLAW: In a medium bowl, whisk together olive oil, vinegar, and salt and pepper to taste. Add shredded cabbage, carrots, mango, scallions, serrano pepper, and cilantro. Toss together, cover, and refrigerate 1 hour to overnight to soften.
AREPAS: Combine warm water and salt in a bowl. Stir to dissolve. Add masarepa and stir to form a dough. (It should have the consistency of soft Play-Doh.) Knead together, divide dough in half, and place each half on a piece of plastic wrap. Place another equal-sized piece of plastic over each and press to flatten slightly.
With a rolling pin, gently roll a dough portion between plastic to form a square, roughly 9-by-12 inches in size and 1/3-inch thick, using plastic to contain it. If it becomes too thin or cracks, fold it back on itself and roll again, until the rectangle is even and smooth. Repeat with second half of dough.
Use a 3-inch round cookie or biscuit cutter to cut eight circles from each dough rectangle (push together scraps and re-roll if necessary).
Top four rounds with one fresh mozzarella slice each, trimming if necessary, leaving a ¼-inch edge. Dip a finger in water and wet the edges of each circle. Top with another circle of dough and gently press together to seal. Wrap arepas in plastic wrap, pressing out all cracks between the dough layers to help contain the cheese.
TO COOK AREPAS: Heat a skillet lightly coated with oil over medium heat. Unwrap arepas and place four on skillet, frying 2 to 3 minutes per side, or until light brown. Don’t worry if they crack slightly. Remove from skillet to a plate or sheet pan, and repeat with remaining arepas.
To serve, plate arepas with slaw on the side. Pass around with your favorite hot sauce.