Blanch potatoes until al dente; strain and set aside. While potatoes are cooking, heat 1 tablespoon olive oil in sauté pan. Add sirloin and sauté for 5 minutes. Add scallions and cook for 1 minute. Add tomatoes and cook for 1 minute. Add potatoes and stir in cumin. Cook for 3 minutes, stirring occasionally. Cool in refrigerator.
Crush garlic with 1 teaspoon olive oil and place in large bowl with cornmeal, salt, pepper and parsley. Pour in hot stock and stir with wooden spoon until ingredients are well combined and the dough is elastic. (Don't overcook the cornmeal or it won't stick together.) Set dough in refrigerator for about 5 minutes - it will become less sticky as it cools. Cover large pastry board with plastic wrap and set dough on top; cover with another sheet of plastic wrap. Roll dough out through the wrap to 1/8 inch thick. Using a 3 inch diameter glass, cut 16 to 18 disks through the wrap. (The wrap shouldn't tear.) Peel top wrap off disks and brush disk perimeters with egg wash. Place a teaspoon of the beef stuffing on the lower half of each disk; grab the bottom sheet of plastic wrap and fold it up (the dough will fold with it) to create each half-moon empanada. Use edge of glass to seal each empanada through the plastic. Remove each empanada from the plastic. Heat 2 to 3 inches of oil in medium saucepan; when hot drop several empanadas in at a time and cook until pale gold. Remove and drain on a wire rack. Serve with Aji Sauce.