My mom's ABC soup only has 4 ingredients - pork ribs, potato, carrot, and onion. I tweaked her recipe to add in the vegetables a little later so that they are tender but not mushy.
Rinse pork ribs and trim off excess fats (if any). Place in a large pot and fill with cold water, enough to submerge the pork ribs. Cover the pot, set over high heat and bring to a boil. Keep the water boiling for 3 to 5 minutes - you will a greyish layer (the scum) forming on the surface. Drain the pork ribs and give them a good rinse to get rid of the scum. Rinse the pot.
Place the rinsed pork ribs in the clean pot and add in 2.5 litres of water. Add in a pinch of salt and bring to a boil. Turn down the heat and simmer for 30 minutes.
Prepare the vegetables while soup is simmering: peel and dice the onion into 1/2-inch dices. Peel and dice the potatoes into 6 to 8 pieces each. Lastly, peel and dice the carrot into chunks, slightly smaller than the potatoes.
After 30 minutes of simmering, add in all the vegetables. Continue to simmer for another 30 minutes, until vegetables are tender. Season with salt and white pepper to taste and serve.
NOTES
My mom chose to use pork ribs instead of pork bone because there's meat on them and that pork bone is pretty expensive here in Singapore. You can substitute with chicken bones but the soup will taste lighter.
Boiling the pork ribs first will help to expel the scum, dirt and excess oil. This will give you a cleaner-tasting and less oily soup. I do this every time I use pork ribs / pork bone for soups.
It doesn't matter which type of onion you use, as they will all soften and "melt-in-the-mouth" after all the simmering. All the sweetness will go into the soup so you won't taste any of the sharp bite in the onion. I used red onion as it's cheaper in Singapore.
I love using medium-starchy potatoes for this recipe as I find that starchy potatoes don't hold their shape well - they disintegrate too easily and will taste too floury and soft.
The carrot is diced into smaller pieces than the potatoes as carrot takes longer to cook.