Rinse oxtail under cold, running water and with a knife, trim excess fat. In a deep pot, place oxtails and enough water to cover. Over medium heat, bring to a boil, skimming any scum that accumulates on top.
Lower heat, cover, and simmer, adding more water as needed to maintain 7 cups, for about 2 to 3 hours or until oxtails are easily pierced with a fork. Drain oxtail, reserving broth.
Peel and discard the outer, fibrous skin layers of banana heart until you reach the lighter, softer core. Trim off the stem and cut the banana core into fourths. In a bowl of cold salted water, place cut banana and soak for about 15 to 20 minutes. Using hands, squeeze to release bitter sap. Rinse with cold water and drain well, discarding liquid.
In a pot over medium heat, bring about 4 cups water to a boil. Add banana heart and blanch for about 1 minute. With a slotted spoon, remove from liquid and set aside.
Add eggplant and blanch for about 1 minute. With a slotted spoon, remove from liquid and set aside. Add long beans and blanch for about 1 minute. With a slotted spoon, remove from liquid and set aside. Add pechay and blanch for about 30 seconds. With a slotted spoon, remove from liquid and set aside.
In a skillet over medium heat, add rice flour and cook, stirring occasionally, until lightly browned and toasted.
In a bowl, combine toasted rice flour, annatto powder, and 1 cup of the reserved broth. Whisk until smooth and well blended. Set aside.
In a large pot over medium heat, heat oil. Add onions and garlic and cook until limp and aromatic.
Add oxtail and cook, stirring occasionally, until lightly browned. Add fish sauce and cook for about 2 to 3 minutes.
Add the remaining 6 cups of reserved broth and bring to a simmer.
Add rice flour mixture and peanut butter, whisking well to prevent lumps.
Season with salt and pepper to taste. Continue to simmer for about 7 to 10 minutes or until it begins to thicken slightly.
Add blanched vegetables to the kare kare and cook for about 2 to 3 minutes or until heated through and tender yet crisp.