Cut pork belly into 2-inch cubes, ensuring each piece has skin attached. Season generously with salt, black pepper, and cumin. Let rest for 15 minutes. In a large heavy pot or Dutch oven, place pork pieces skin-side down with 1 cup water. Bring to boil over medium-high heat, then reduce to medium. Cook uncovered for 30-35 minutes until water evaporates and pork begins frying in its own fat. Turn pieces occasionally to ensure even browning. Continue frying for 10-15 minutes until skin is deeply golden and crispy. Meanwhile, peel and cut sweet potatoes into thick rounds. Boil in salted water for 15-20 minutes until tender but not mushy. Drain and keep warm. For salsa criolla, slice red onions thinly and soak in cold water with lime juice for 10 minutes to reduce sharpness. Drain and mix with sliced aji amarillo, cilantro, salt, and additional lime juice. Let sit for 5 minutes. Remove chicharrón to paper towels to drain excess fat. Arrange sweet potato rounds on plates, top with chicharrón pieces, and serve with generous portions of salsa criolla on the side.