Add glutinous rice into a small bowl and rinse it 2 to 3 times. Add water to cover rice with 1 centimeter (0.5 inch) excess. Let the rice soak at room temperature for 4 to 5 hours or in the fridge overnight (cover / wrap the bowl if placed in the fridge).
Drain the rice. Add white sugar and mix well.
Wash lotus root carefully. Peel the rind off and discard it. When you cut off the tough ends, be careful not to cut off so much that you reveal the tubes.
Cut about 2 centimeters (1 inch) from one end of the lotus root. Save the small piece that you cut off to use as a lid.
To stuff the lotus root with glutinous rice, add a spoonful of rice onto the cut end and use a chopstick to push the rice into the holes. Use a chopstick to squeeze the stuffed rice and compress it. Carefully fill up all the tubes, including the small ones. (It will take some time. I highly suggest you sit down and play your favorite TV show while doing this.)
Place the small “lid” back to cover the cut end, in exactly the position it originally was (so the glutinous rice will expand into the holes of the “lid” during cooking). Use 6 to 8 toothpicks to seal the “lid”. You should push the toothpicks as far in as you can. The glutinous rice will expand during cooking and the “lid” will fall off or be pushed apart if not sealed carefully.
Add lotus root to a small pot, with 2 cups of water to cover (*see footnote 2). Add rock sugar (or white granular sugar) and brown sugar. Bring to a boil over high heat. Stir during boiling to dissolve the sugar.
Turn to low heat, cover, and boil for 1 hour. Keep the water boiling throughout. Flip the lotus root after 30 minutes and add more water to cover it, if necessary.
Turn to lowest heat to simmer for another 20 minutes. Flip the lotus root 4 to 5 times during cooking.
To serve after cooking, transfer the lotus root to a cutting board and allow it to cool. Cut into evenly thick slices when the root is cool enough to handle.
To serve later, leave the lotus root in the pot until it cools off completely before slicing. If you plan to serve it the next day, place the lotus root in an airtight container and add the sugary water to cover. Store in fridge.
Drizzle osmanthus syrup (or honey) on top of the cut lotus root. Serve warm or cold.