Step 1

Heat oven to 325°F. On a rimmed baking sheet, toss tomatoes, garlic and onion with oil and 1/2 teaspoon each salt and pepper. Roast until tomatoes are tender and juicy and onion is tender, 60 to 70 minutes. Transfer all vegetables to a pot along with 4 cups of water; bring to a boil, then in a blender or using an immersion blender, puree until smooth.

Step 2

Heat oven to broil. Arrange baguette slices on a baking sheet, top with Parmesan and broil until melted; serve with soup.