Cook the soba or spaghetti according to the package directions.
Meanwhile, rinse the scallops and pat dry. In a pot, bring the broth and scallions to a simmer. Add the carrots and simmer for 2 minutes. Add the scallops and cook until they are the same color throughout, 5 to 7 minutes. Remove from heat and stir in the soy sauce. Divide the soba or spaghetti among individual bowls. Ladle the soup over the noodles. Serve with additional soy sauce on the side.
Tip: You can vary the vegetables according to your mood or the contents of your crisper. Consider using green beans, snow peas, edamame, bok choy, or leafy greens, such as Swiss chard, in addition to or in place of the carrots.