In a wok, heat the cooking oil.
Soften the onion slices with a little salt and pepper over medium heat.
Add the garlic and ginger. Sprinkle in a bit more salt and pepper. Continue cooking, stirring often, for a minute or two.
Throw in the carrot and potato cubes, and cook with occasional stirring for a minute.
Pour in the broth.
Grate the apple dirctly into the pan and stir.
Cover the pan and cook for 15 to 20 minutes or until both the carrot and potato cubes are cooked through.
Add the roux, a heaping tablespoonful at a time and stirring after each addition. When all of the roux has been added, simmer curry for another 10 minutes.
Taste the curry sauce and adjust the seasonings, as needed.