If You’re Using A High-Speed or Traditional Blender:

1. Preheat & Prep. Start by preheating the oven to 400 degrees F. Lightly grease a sheet pan with oil or spray oil. 
2. Add Tomatoes To Baking Sheet. Spread the tomatoes out on the baking sheet and drizzle with olive oil. Sprinkle with salt and pepper. 
3. Roast Tomatoes. Add the pan to the preheated oven and roast 20 minutes. Stir and roast 15-20 minutes more. 
4. Cook The Onion. While the tomatoes are roasting, melt the butter in a large saucepan on the stove over medium heat. Add the diced onion and saute 5-7 minutes, or until the under is tender and translucent. Add the garlic and cook 1-2 minutes more. Remove the pan from the oven. 
5. Work In Batches To Blend. When the tomatoes are done, add the onion mixture to a blender along with half of the roasted tomatoes, the basil, and the cornstarch. Add enough broth to cover the tomatoes (be careful not to fill more than the max fill line of your blender. You will need to work in batches.) Puree until totally smooth and pour back into the pot you used to cook the onions. Repeat, as needed, in 2-3 more batches until all the tomatoes are blended well.
6. Simmer To Thicken. Bring the soup to a simmer over medium heat and stir until the soup has thickened slightly. 
7. Finish The Soup. If desired, stir in cream and garnish with more basil. Serve warm. Store leftover tomato soup in an airtight container or freeze up to 2 months. Thaw frozen soup in the fridge, then heat on the stove. If the soup has separated at all, you can blend it again a bit with an immersion blender or traditional blender to pull it back together quickly. 

If You Only Have An Immersion Blender:

1. Follow the instructions above until step 4. 
2. Combine. When the tomatoes are done roasting, scrape them and any pan juices into the pot with the cooked onion and garlic. Pour in the broth (and cornstarch, if using).
3. Blend. Use an immersion blender to puree the soup until smooth. (It’s safer to blend cool soup than hot soup!)
4. Simmer To Thicken. Heat the soup over medium heat bring to a simmer, stirring until thickened slightly.
5. Finish The Soup. If desired, pour in heavy cream and garnish with additional basil. Serve warm. Store leftover tomato soup in an airtight container or freeze up to 2 months. Thaw frozen soup in the fridge, then heat on the stove. If the soup has separated at all, you can blend it again a bit with the immersion blender to pull it back together quickly.