Pierce the butternut squash a few times with a knife. Microwave on high for 4 to 6 minutes, adding 1 minute at a time, until it can be pierced easily with a knife but still holds its shape. Alternatively, wrap the whole squash in foil and bake at 375°F for 30 to 40 minutes until just tender. Let cool until easy to handle, then halve, scoop out the seeds, peel, and cut the flesh into bite-sized chunks. Set aside. (This step can be done ahead of time.)

Bring the 4 cups of water to a rapid simmer in a large soup pot. Add the soba or udon noodles and cook according to package directions until al dente, about 4 to 5 minutes.

Add the grated ginger, bouillon cubes, thawed peas, and squash chunks to the pot. Stir to dissolve the bouillon and cook for 3 to 4 minutes over medium heat. Add up to 1 cup additional water if the soup seems too thick.

Stir in the spinach and cook just until wilted, about 1 to 2 minutes. Add the scallions and stir to combine.

In a small bowl, whisk the miso paste with ⅓ cup warm (not boiling) water until smooth. Stir into the soup. Taste and add more dissolved miso if desired — do not boil the soup after adding miso, as heat destroys the beneficial probiotics. Season with black pepper and serve immediately.