Preheat oven to 350°F. Melt butter in a Dutch oven over medium heat. Add onion and garlic; cook, stirring occasionally, until softened, about 5 minutes. Add parsley, tomatoes and 1 cup water. Season with salt and pepper.
Increase heat to high and bring to a boil. Reduce heat, cover and simmer, stirring occasionally, until tomatoes are very soft, 15 to 20 minutes. Let cool. Top bread slices with cheese, place on a baking sheet and bake until cheese is melted and lightly browned, 12 to 15 minutes.
Remove parsley sprigs; discard. Puree soup in a blender in batches and return to Dutch oven. Gently reheat. Ladle into bowls, rest a crouton on top of each; serve hot.