In a large mixing bowl, combine the panko, egg, garlic, 1/4 tsp cayenne, salt, and pepper. Next, add in the ground beef and mix throughly with hands.
Shape the beef mixture into 12, 1-inch sized meatballs.
Preheat a large skillet to medium heat. Add the olive oil and cook the meatballs, turning occasionally until brown and no longer pink inside. Transfer the meatballs to a plate.
In a small mixing bowl combine the apple juice, soy sauce, brown sugar, cornstarch, ginger, and remaining cayenne pepper. In the same skillet as you cooked the meatballs, add the apple juice mixture.
Bring the sauce to a slight simmer and continue to cook for about 2 minutes or until slightly thickened. Return the meatballs to the skillet to heat through and coat with sauce.
Transfer meatballs to serving dish and garnish with green onions.