Cook the tapioca pearls following the instructions on the package.
After cooking, rinse the tapioca pearls under cold water and drain them. Return them to the saucepan, add the brown sugar, and pour in enough water to submerge them.
Simmer over medium heat for another 5 minutes until the syrup has thickened. Set aside.
Add the fresh watermelon to the blender, followed by the frozen watermelon. If desired, you can also add sugar for extra sweetness.
Blend until smooth and serve with a generous portion of tapioca pearls.
Notes
Fresh watermelon: Choose a fresh watermelon that's sweet and juicy. Sugar is optional, but you can add it if you prefer a sweeter taste or if the watermelon is not sweet enough for your liking.
Frozen watermelon: Incorporating frozen watermelon instead of regular ice cubes enhances the refreshing watermelon flavor of your slushie and creates a more concentrated profile.
Boba or Tapioca Pearls: Choose quick-cooking boba that require about 5 minutes to cook. Sago, gulaman (agar-agar), or grass jelly can also be used as an alternative ingredient.
Brown sugar: You can use light or dark brown sugar to make the syrup.