Heat a Dutch oven or pot over medium heat. Add 1 tablespoon oil and the onion; cook until translucent and edges begin to brown, about 2-3 minutes.

Add garlic, tomato paste, and Worcestershire sauce, cooking until fragrant, about 1 minute. Stir in curry powder, salt, and sugar; cook for 1 minute.

Stir in potatoes, carrots, and chicken stock. Bring to a simmer, cover, and cook until potatoes and carrots are tender, about 20-25 minutes.

Meanwhile, butterfly the chicken breast to make them thin and even. Pat chicken dry, season with salt and pepper, and dust with flour.

Beat the egg in a shallow bowl. Place panko breadcrumbs in another shallow bowl.

Add a thin layer of oil to a cast iron pan or nonstick skillet over medium heat. Dip chicken into egg, then coat in panko.

Cook chicken in batches in the hot oil, about 6 minutes on one side or until golden, flip and cook another 5-6 minutes. Transfer to a plate and repeat with remaining chicken.

Mix cornstarch with water to make a slurry and add to curry sauce. Stir until thickened, then stir in butter. Taste and adjust seasoning if needed.

Slice chicken and place over steamed rice. Ladle curry sauce on the side.