In a large pot, heat the vegetable oil over medium heat.
Add the finely chopped onion and minced garlic. Sauté until the onion is soft and translucent.
Add the diced potato to the pot and cook for about 5 minutes, stirring occasionally.
Add the chopped spinach or Swiss chard to the pot. Cook until the greens are wilted.
Pour in the water or vegetable broth, and bring the mixture to a boil.
Reduce the heat to low and let it simmer for about 10-15 minutes, or until the potatoes are tender.
Stir in the milk, ground black pepper, salt, and ground turmeric (if using).
Allow the soup to simmer gently for another 5 minutes, stirring occasionally. Do not let it boil.
For added richness, stir in the butter until it melts and is well incorporated into the soup.
Adjust the seasoning with additional salt and pepper to taste.
Ladle the jaju into serving bowls.
Garnish with freshly chopped cilantro or green onions.
Enjoy your meal!