Combine beef, shallots, garlic, salt, and pepper.

Carefully press meat mixture into the onion slices to fill them.

Dredge in flour, then in egg mixture, then in panko.

Heat two inches (five centimeters) of oil in a large pot and fry in batches until fully cooked, about five to seven minutes.

These can be frozen and reheated in a single layer in a 400-degrees-Fahrenheit (200-degrees-Celsius) oven.