The night before, rinse the red beans. In a large bowl, soak the beans with room temperature water overnight (or for at least 8 hours).
Next day, if using dried mandarin orange peel, rinse under tap water. In a small bowl, soak the peel with room temperature water for 30 minutes. Once soaked, using a spoon, scrape off the white pith from the peel. The pith is bitter.
In a nonstick medium pot, add 6 1/2 cups of water. Bring to a boil, add soaked red beans and soaked mandarin orange peel. Cook over medium heat for about 1 hour, stirring often.
In the meantime, in a medium bowl, combine glutinous rice flour and half of the warm water. Using a clean hand, mix until the dough just comes together. Add more water if needed. Knead for at least 4 minutes. The dough should be smooth and with very little stickiness. Divide into 15 portions (about 6 grams each). Roll each portion into a small ball. Place them on a large freezer-safe plate with space in between. Chill in the freezer until frozen.
Prepare a large sieve over a large bowl. Using a small sieve, transfer 1/2 cup of red beans into a big sieve. Press the beans with a spatula or wooden spoon. The mashed beans would strain through and leaving the outer skin in the sieve. Transfer the outer skin to a medium bowl. Continue with the rest of the red beans. Keep the liquid and dried orange peel in the pot.
Add the mashed beans back to the pot (if desired, add some or all of the outer skin back as well). Add 4 1/2 cup water. Cook for another 45 minutes.
Add more water if needed. Add 60 grams of rock sugar and salt in the red bean soup. Taste and adjust with more rock sugar if needed. If using fresh orange zest, add them right now. If using mandarin orange peel, remove the peel now. Cook for another 15 minutes and the sugar is melted, it’s ready.
Meanwhile, bring a small pot of water to a boil. Add desired amount of frozen rice balls. Cook for about 5 minutes.
Transfer the red bean soup into serving bowls, add cooked glutinous rice dumplings. Serve hot.