1. Heat the oil over medium flame. Add the spring onion and simmer until the onion is translucent.
2. In a large sauce pan, add the water, potatoes and a tsp. of salt. Bring to a boil, then remove the foam that forms on the top.
3. Stir in the cooked onions and milk. Bring to a boil, then add the cheese, salt, pepper and bijol. Reduce heat to low and simmer until potatoes are soft and falling apart (approximately 40 minutes).
4. Mash the potatoes a bit with the back of a spoon to thicken the soup. Leave the soup a little chunky.
5. Add salt and pepper as needed. Serve the soup with a few slices of avocado in each soup bowl.