Place the pounded pork in a zip lock plastic bag. Add the onion, scallion, cumin, garlic, salt and pepper. Refrigerate for 1 hour or overnight. Be sure the steaks are evenly covered.
Place the pork pieces between sheets of wax paper, then pound until each steak is about ¼ inch thick.
In a large skillet, heat the olive oil over medium heat. Place the pork into the skillet and cook for 2 minutes per side. Add the mushrooms and cook for 3 minutes more or until the pork and mushrooms are cooked. Season with salt and pepper and set aside.
To make the sauce, In a blender, mix all the ingredients until smooth.
To assemble: Top each arepa with pork and mushrooms. Drizzle avocado sauce on top and garnish with fresh cilantro. Serve warm.