How to Make It
Heat oven to 400°F.
In a large roasting pan, combine the carrots, parsnips, onion, 3 tablespoons of the oil, 1½ teaspoons salt, and ¼ teaspoon pepper. Spread the vegetables in an even layer and roast, stirring occasionally, until tender and golden brown, about 45 minutes.
Meanwhile, arrange the bread slices on a baking sheet and brush with the remaining oil. During the last 10 minutes of roasting time, toast the slices until crisp.
Transfer the vegetables to a blender and purée with 3 cups water, adding more water if necessary, ¼ cup at a time, until smooth. Rewarm in a pot over medium-low heat, if necessary. Divide among individual bowls and serve with the olive oil toast.