Heat oven to 400° F.
In a large roasting pan, combine the carrots, parsnips, onion, 3 tablespoons of the oil, 1 1/2 teaspoons salt, and 1/4 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 puree with 3 cups water, adding more water if necessary, 1/4 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.