Preheat oven to 425°F. Arrange tomatoes cut-side up on a baking sheet. Scatter onions and garlic over the tomatoes. Drizzle with olive oil and season with salt and pepper. Roast for 35-40 minutes until tomatoes are softened and slightly charred.
Transfer the roasted tomatoes, onions, and garlic, along with any pan juices, into a large pot. Add vegetable broth, basil, and oregano. Bring to a simmer over medium heat and cook for 20 minutes.
Using an immersion blender, purée the soup until smooth. Alternatively, carefully transfer to a blender in batches and purée. Adjust seasoning with salt and pepper.