For the guacamole: In a medium bowl, mash together the avocado, lemon juice and salt with the back of a fork. Set aside.
For the sandwiches: Split the potato buns. Place a slice of smoked mozzarella on the bottom of each bun. Divide the avocado spread among the bun tops.
Preheat a grill pan over high heat. Meanwhile, place a piece of plastic wrap over the beef fillets and pound them to about 1/6-inch thick. Sprinkle the fillets evenly with the cumin and salt, then drizzle with the oil. Place the fillets on the hot grill pan and cook until deep brown grill marks form, about 2 minutes. Flip and continue to cook for an additional minute, until just cooked through.
Transfer the hot fillets to the buns. Top with a few leaves of arugula and cover with the bun tops.