Put the chopped onion in a mug with 1 tablespoon of water and cover with a saucer. Microwave on high for 3 minutes.
An adult must remove the mug as it will be very hot.
Put all the ingredients including the cooked onion (but not the sour cream and mashed potatoes) into a rice cooker and press start. When the cooking cycle finishes, allow the goulash to remain on warm for another 10 minutes before an adult removes the inner pan from the cooker.

Warm the left-over mashed potato in a covered bowl in the microwave for 1 minute. Stir. Pour the goulash on top and serve with sour cream.