Set timer for 25 minutes. Place water, thinly sliced onion, diced tomato and chicken stock cube into a pot and set on a stove on high heat. Cover the pot and let the water boil.
Meanwhile, peel and chop up the carrots and potato. Carrots can be halved and then chopped into smaller pieces and potatoes can be halved and then quartered.
When the water has boiled, give it a stir and then add carrots in. Partially cover the pot and leave to boil for 10 minutes (for the carrots to soften).
Stir the soup again and then add the potato pieces in. Partially cover the pot, leaving the soup to boil. Soup is ready when timer goes off (or once your potato softens. I like to boil mine for a bit more then top up with hot water when about to eat).
Ladle into a big bowl and enjoy with steamed rice.