Cook the sago pearls in boiling water until translucent, then rinse with cold water. Be sure to add the sago when the water is boiling; if you add them too soon, they will disintegrate.
Drain the sago and nata de coco, and combine them in a large bowl.
Dice the "cheeks" of the mangoes and add them in. Set the seeds aside.
In a separate cup, combine half-and-half and condensed milk (more to taste, depending on the sweetness of the mangoes).
Slice off the flesh from the seeds and add it to the mixture. Blend until smooth.
Gently stir the sauce with the sago, nata de coco, and mangoes.
Refrigerate for a few hours before serving.
Notes
Mangoes: Use ripe Manila or Ataulfo mangoes. You can use another type of mango that is sweet and not stringy, like the Haden variety.
Sago: Use small-size sago pearls as they tend to cook faster. You can also use tapioca pearls which are similar and cooked the same way.
Nata de coco (or coconut gel): Made from fermented coconut water, it is chewy, sweet, and jelly-like. It is an optional ingredient also used in Halo-Halo (Iced Shaved Dessert).
Half-and-half: I used this homogenized blend of cream and milk. Alternatively, you can combine equal parts of heavy cream and milk. Using heavy cream or all-purpose cream results in a thicker and creamier sauce. You can also use evaporated milk or coconut milk.