In a bowl, mix the ground turkey with the milk, onion, ¼ cup panko, egg, salt, and pepper. The mixture will be very loose. At this point, it would be best to put them in the fridge to firm up. If you feel like the mix is still to difficult to shape, you can add more panko to the mix, but the more panko you add the more dense your menchi katsu will be. Loose filling equals juicer, looser patties.
Divide the meat mixture into 2 equal portions and shape into patties. Dip into the flour, shaking off excess, then into the egg, then the panko, being sure to coat fully and evenly. Place the patties in the fridge while you heat up oil for deep frying.
In a very deep heavy bottomed pot, heat up 2 inches of oil over medium high heat until 350°F. Carefully lower in a menchi katsu and cook, flipping as needed, until deeply golden, crunchy, and cooked through, about 5 minutes. Remove from the oil and drain on a wire rack. Top with a slice of cheese. Deep fry the other patty then top with a slice of cheese.
Spread mustard (I used Japanese mustard) on the bottom bun. Top with a cheesy menchi katsu patty and drizzle on sauce to taste. Top with the top bun and enjoy immediately!
