For katsu sauce, heat oil in a saucepan over medium-high heat, add onion, ginger and garlic and sauté until tender (3-4 minutes). Add curry powder, stir until fragrant, then stir in flour and cook until sandy (1-2 minutes). Gradually whisk in stock, add soy sauce and honey and simmer, whisking occasionally, for about 10 minutes. Stir in rice vinegar and garam masala and keep warm.
Meanwhile, preheat oven to 220C. Dip chicken in flour, then egg, then panko crumbs, shaking off excess in between. Heat oil in a large frying pan and fry chicken, turning occasionally, until golden (3-4 minutes). Transfer to oven until cooked through (3-4 minutes). Serve with steamed rice, and topped with katsu sauce and spring onion.