Roast tomatoes: Heat oven to 425 degrees. Arrange tomatoes in a baking dish. Drizzle with oil and season with salt and pepper. Roast until tomatoes begin to burst, 20 to 25 minutes. Scrape tomatoes and any juices into a bowl and let cool.
Heat broiler. Place mozzarella on sliced halves of bagel and broil until cheese is melted and bread is toasted. Top with roasted tomatoes and arugula.