Place the turnips, mushrooms, and turmeric into a large soup pot, cover with 4 cups of water, and bring to a boil. Reduce heat and bring to a simmer, then cook for roughly 15 to 18 minutes, until turnips are tender, but still have a bit of a bite. Add the miso paste to a bowl, ladle 1 cup of broth from the soup pot into the bowl, and whisk until dissolved. Return mixture to soup pot, taste, and adjust by adding more miso directly to the broth, if need be.
In a separate pot, bring roughly 3 quarts of water to boil, add soba noodles, and cook according to instructions on the package. Drain and rinse with cold water, use your fingers to separate noodles if need be.
To serve, divide the noodles among soup bowls and ladle the vegetables and broth over the top. Finish with a sprinkling of sliced onions and microgreens.
To store, combine leftover soup and noodles in a container with a fitted lid. To reheat, gently bring the soup to desired temperature; if heated too fast, on too high of a flame, miso can lose some of its nutritional integrity.
