Nonstick vegetable oil spray
1 large onion, thinly sliced
2 pounds plum tomatoes, halved lengthwise
1 pound carrots, peeled, cut into 1/2-inch-thick rounds
2 garlic cloves, unpeeled
1 tablespoon olive oil
2 1/2 cups water
2 3/4 cups (about) low-fat (1%) milk
1/2 cup thinly sliced fresh basil
Preheat oven to 400°F. Spray large rimmed baking sheet with nonstick spray. Arrange onion, tomatoes, carrots and garlic cloves on prepared baking sheet. Drizzle with oil. Sprinkle with salt and pepper. Roast until vegetables are tender and brown, turning occasionally, about 55 minutes. Cool slightly. Peel garlic cloves. Transfer vegetables to large bowl (do not clean baking sheet).
Add 1 cup water to baking sheet, scraping up browned bits; add to blender, then add half of vegetables and puree until smooth. Transfer to large saucepan. Add remaining vegetables and 1 1/2 cups water to blender and puree. Transfer to same saucepan. Gradually add enough milk to soup to thin to desired consistency. Stir in 1/4 cup basil. Simmer 10 minutes to blend flavors. Season with salt and pepper. (Can be made 6 hours ahead. Cover; chill. Bring to simmer before continuing.)
Ladle soup into bowls. Sprinkle with remaining 1/4 cup basil and serve.