Cook the onions in the oil with ½ teaspoon salt in a large pot over medium heat, stirring occasionally, until softened, 4 to 6 minutes.
Add the tomatoes, chicken broth, and oregano. Simmer until beginning to thicken, 10 to 15 minutes.
Transfer to a blender and puree until smooth. Top with the croutons and Parmesan before serving.