Heat a 12" heavy-based pot or skillet on medium high heat. When the pan is hot, add the vegetable oil to coat the whole surface of the pot/pan.
Add the onions and stir-fry for 2 minutes. Then add the meat and stir fry for another 2 minutes.
Add the carrots and potatoes, and stir fry for 1 minute.
Add 1-1.5 cups of water, mix the contents in the pot, bring to a boil, and turn the heat down to medium-low.
Close the lid. (Note: I like to steam the chicken is as little liquid as possible to retain the flavor of the chicken in the meat. This technique requires a bit more attention. If you aren't particular, just add 3 cups of water in directly).
Set the timer to 12 minutes. In case your heat is too high, periodically check the water every 5 minutes and add another 1/2-1 cup of water in if necessary.
When the timer rings, open the lid and add the 3 curry roux cubes. Mix the roux and stir the contents constantly, adding more water to get the consistency you like. Bring to a boil, about 3-5 minutes.
Turn off the heat, and serve the curry on a bed of cooked, short to medium-grain steamed rice.