Heat oven to 400 degrees F. In a large roasting pan, combine the carrots, parsnips, onion, garlic, olive oil, salt, and pepper. Spread the vegetables in an even layer and roast, stirring occasionally, until tender and golden brown, about 1 hour.
Transfer the vegetables to a blender and puree with vegetable broth, 1/2 cup at a time until smooth. (Depending on blender size, you might have to do part at a time and then switch.) Rewarm in a pot over medium- low heat.
Garnish with green onions and serve with fresh bread.