Put 2 tablespoon Olive oil into a saucepan and add 1 Onion, 2 Garlic cloves and 1 tablespoon Fresh ginger. Cook over a low heat until the onion has softened, about 5 minutes.
Once the onions have softened, add 2 tablespoon Curry powder and 1 teaspoon Turmeric. Mix well.
Add 1 tablespoon Plain flour and mix again. Gradually add 280 ml Chicken stock, 120 ml Coconut milk and 1 teaspoon Soy sauce, mixing continuously.
Simmer for 5 minutes. You can leave your sauce as it is, or blend for a smoother consistency.
Notes
Make sure your spices haven't been left open for longer than 6 months, as you want them to have the best flavour.
You can make this spicier by using a madras curry powder.
If you want a really smooth sauce, then you can strain it after blending it.
If you wanted to hide some vegetables in this katsu curry recipe, then you can simmer some carrots with it and then blend them in when they are fully softened.
You could also stir some creamy peanut butter into it to make it more like a satay and add some protein too.