Soak the mung beans: Place the mung beans in a medium bowl and cover with a few inches of water. Leave on the counter for at least 8 hours until fully soaked and plump. Drain and rinse well. Set aside.
Cook the mung beans: In a medium pot, add the drained mung beans and cover with about 2 inches of water. Add a big pinch of salt and mix well. Place over high heat and cook, uncovered, until at a boil. Scrape off any white foam that appears and continue to cook until the beans are tender. (This should take around 20 minutes but give them a taste to see. You want them to be tender while still holding their shape.) Once done, turn off the heat and set aside.
Bake the rice: While the mung beans are cooking, follow the instructions for my Easy Baked Rice to make one batch using Calrose Botan Rice as the variety. The total bake time will be 35 minutes. Once done, set aside until ready to serve. Cook the onions and garlic: Place a large pot over medium-high heat and drizzle in the oil. Once hot, add the onions plus a pinch of salt. Mix well and cook for 10 minutes, stirring every so often, until soft and tender. Then add the garlic, red pepper flakes, and black pepper. Mix well and cook for about 5 minutes, stirring often, until the garlic is soft and fragrant.
Add the mung beans: Transfer the cooked mung beans with the cooking liquid into the large pot of cooked onions and garlic. Depending on how much cooking liquid there is, you may need to add some more water to get the right consistency. You want it to be a hearty, brothy soup but not too liquidy. Find a balance that feels good to you by adding an additional cup of water at a time.
Bring to a boil and season: Once everything is combined, place over high heat and bring to a boil. Once at a boil, reduce the heat to low and add the vinegar and a splash of tamari. Give the soup a taste and add more of each, if desired.
Serve the soup: Once the soup is seasoned to your liking, grab a handful of desired greens. If they are larger pieces, roughly tear them. Add them directly to a bowl. Then add a scoop of cooked rice and a big pinch of sliced green onions. Pour the hot soup over everything and mix. Enjoy immediately!