Place the onion, garlic, red bell pepper, scallions, cumin and achiote in a blender and blend until smooth.
In a medium pot over medium heat, place the pork bones, water and onion mixture and cook for about 30 minutes. Taking out the foam from the pot every 15 minutes.
Add the potatoes and season with salt and pepper, increase the heat to medium-high, and bring to a boil. Reduce the heat to low, cover, and gently simmer until the potatoes are soft, approximately 40 minutes.
Taste and adjust seasoning if desired. Sprinkle with fresh cilantro and serve immediately with avocado and ají on the side.