Heat oil in a large pan over medium heat. Add the onion, carrots, potatoes, ginger, and garlic and sauté for 5 minutes. Add the flour, butter, and all the spices and stir to coat. Cook for a minute. Pour in the chicken stock, soy sauce, oyster sauce, and Worcestershire sauce. Cook until the vegetables are completely soft and the sauce has thickened.
Blend everything until completely smooth. Return to the pan and cook for a further 5 to 10 minutes. Taste and adjust seasoning. Keep warm.
Slice the chicken breasts in half horizontally to create four thinner pieces. Pound each piece thin and even. Season with salt, pepper, and smoked paprika on both sides.
Set up three bowls with seasoned flour, beaten seasoned eggs, and panko mixed with sesame seeds. Coat each piece in flour, then egg, then press firmly into the sesame panko mixture.
Heat oil in a wide pan over medium high heat. Shallow fry the schnitzels in batches for 3 to 4 minutes per side until deeply golden and crispy. Drain on a wire rack.
Serve steamed rice on a plate or in a bowl. Slice the schnitzel and place alongside or on top of the rice. Pour the warm katsu curry sauce generously over the top. Garnish with scallions, a drizzle of chilli oil, and a lemon wedge on the side.