Heat oven to 400. Coat a shallow roasting pan with cooking spray; cook potatoes (covered with foil) for 15 minutes.
Uncover and roast until tender, about 30 minutes. Meanwhile, bring carrots, onion and 1/2 cup water to a boil in a medium nonstick skillet.
Reduce heat; simmer, covered, 5 minutes. Uncover; cook until water evaporates and vegetables are tender, about 3 minutes.
In a food processor, process potatoes with remaining ingredients until smooth. Add just enough water to make a spreadable paste, about 1 cup.
Add carrot mixture; pulse until blended.
Serve at room temperature or chilled, with, bagles, challah or pita wedges. Makes about 4 cups.