1. Season the fish fillet with salt and pepper. Bake it in the oven at 180 degrees for 10 minutes or until it is half cooked. Remove from oven and set it aside.
2. Using a sauce pan, melt the butter and sauté the garlic until it is golden yellow colour. Add in soy sauce, dry mustard and ketchup.
3. Gently simmer for a minute on low heat. Loosen the sauce with water and drizzle in some lemon juice. Adjust seasoning if necessary.
4. Spoon the sauce onto the fish. Bake it in the oven again for 15 minutes or until it is fully cooked. The cooking times will vary depending on the size of your fish fillet.
5. Serve when it is piping hot!