Bring a large pot of water to a boil. Add in the sago and stir immediately. Lower the heat to a simmer and cook sago for 10 minutes. Remove from heat, cover, and let sit for 10 minutes until the sago becomes completely translucent throughout.
Pour the cooked sago into a fine mesh sieve to drain away the water. Fill the same pot with cold water and place the sieve on top. Keep the sago submerged in cold water until ready to use.
In a blender, add the mango chunks, coconut milk, and condensed milk. Blend until smooth. Taste and add more condensed milk if you’d like it sweeter.
Pour the mango mixture into a large serving bowl. Add in the drained sago and stir to combine.
Serve the mango sago in small bowls or glasses. Garnish with more coconut milk and fresh mango chunks if desired.