Cook rice noodles according to package directions. Drain and set aside.
In a large wok or skillet over medium-high heat, add 1/4 cup vegetable broth. Add garlic and simmer for 1 minute. Then add onion and cook for 5-6 minutes, stirring occasionally, until onion is translucent. Add remaining 3 1/2 cups of vegetable broth, soy sauce, and sweet potatoes. Turn heat to high. Once starting to boil, reduce heat to a rolling boil (more than a simmer but not heavy boiling) and boil for 14-15 minutes, until sweet potatoes are almost soft throughout, stirring occasionally.
Next, add broccoli and bok choy and simmer for 5 more minutes.
Meanwhile, in a small bowl, combine 1 tablespoon amaranth powder (or cornstarch) to 3 tablespoons water. Stir and blend into sweet potato mix in wok. Then add peanut butter, ginger and cayenne pepper. Stir until combined. Simmer for 1 minute, stirring. Test the sweet potatoes for doneness and add a couple minutes of boiling time if necessary.
Remove from heat and add noodles to pot, then lime juice. Stir to combine. Serve in bowls and top with green onions, fresh cucumber, cilantro or peanuts.
Store in an airtight container in the fridge for 2-3 days.