Step 1Steam potatoes and squash until vegetables are tender, about 45 minutes. When cool enough to handle, scoop squash from shell and mash enough to measure 1/4 cup. Transfer to bowl large enough to make dressing. Let cool. Cut steamed potato wedges into quarters.
Step 2Cook carrots in boiling water just until tender, about 5 minutes. Drain, place in bowl of ice water until cold, then drain again.
Step 3Toss together potatoes, carrots, cucumber and green onion.
Step 4Spoon mayonnaise, sour cream, sugar, vinegar, mustard, salt and pepper to taste into bowl with squash puree and stir until all ingredients are blended. Stir dressing into potatoes to coat. Cover and chill at least 30 minutes before serving.