Soak the mung beans overnight if possible. If it is not, just plan to cook them for longer. Strain the beans and add the water to a saucepan. Bring up to a boil and reduce to a simmer for about 45-75 minutes, depending on the soaking time and age of the mung beans. A pressure cooker gets the job done much faster, if that equipment is available to you.
Meanwhile, chop the sweet potatoes into a dice. Add to a skillet over medium heat with a splash of the olive oil. Saute for about 5-10 minutes, allowing the chunks to sear briefly before shaking the pan and flipping. Each side should gain a bit of color before flipping.
While the sweet potatoes cook, chop the rhubarb and mince the garlic. Once the sweet potatoes are barely cooked through, toss in the rhubarb and garlic. Shake the pan periodically until the rhubarb is tender.
In a large bowl, whisk together the dijon, orange juice, vinegar, salt and pepper. Slowly drizzle in the olive oil while whisking. Stack up the greens and thinly slice into shreds.
Strain the mung beans, shaking to remove any excess liquid. Toss the beans into the vinaigrette, gently fold in the sweet potatoes, rhubarb and leafy green shreds.
Taste. Add more salt and pepper if desired.