Preheat oven to 400 and line a cookie sheet with parchment paper and set aside. Remove seeds and core from red peppers and slice into four pieces each. Next, peel the garlic and set on a cutting board. Smash each piece of garlic with the side of a knife.
Add red peppers, grape tomatoes and chopped onion, and garlic to the cookie sheet. Sprinkle with olive oil and top with salt, pepper and red pepper flakes if using. Roast on the center rack for 20 minutes.
While the vegetables are roasting, add the whole peeled tomatoes and red wine vinegar to a large pot. Cover and let simmer.
After 20 minutes remove the vegetables from the oven and add everything to the large pot with the tomatoes and stir to combine. *See recipe notes for optional simmer.
Using an immersion or stand blender, blend the soup until smooth and no chunks remain. Serve warm and enjoy!