Steam sweet potatoes in a large saucepan with a steamer basket and 1 in. water until very tender, about 20 minutes. (If you don't have a steamer, just simmer chunks in the water, stirring occasionally.) Drain, reserving liquid, and let cool slightly.
Whirl ginger in a food processor to mince. Add sweet potatoes, miso, tahini, soy sauce, and enough reserved sweet potato liquid for a creamy texture, about 3 tbsp.; whirl until smooth.
Transfer dip to a bowl and stir in 1 1/2 tbsp. green onions. Sprinkle with remaining 1/2 tbsp. onions and the sesame seeds. Serve slightly warm or at room temperature with bok choy or cucumber slices.
*Find in the refrigerated foods section.
Make ahead: Up to 2 days, chilled.
Note: Nutritional analysis is per 3-Tbsp. serving.