Preheat oven to 400 degrees and line a baking sheet with foil. Place the halved tomatoes on the pan, along with the onion wedges and garlic cloves. Drizzle with the olive oil and season with salt and pepper. Toss to coat.
Bake for 45-55 minutes until the tomatoes are soft and a little browned around the edges. Watch the garlic cloves while roasting and remove if they get too browned.
After the tomatoes, onions, and garlic have roasted, let them cool. Once cool, pour them into a large pot or Dutch oven. (Squeeze out the garlic cloves from their papery skin).
Add the chicken broth, balsamic vinegar, cayenne pepper, dried thyme, fresh basil, and crushed red pepper flakes. Give everything a good stir. Using an immersion blender, blend until super smooth. Or remove to blender and blend in 2 batches until smooth.
Season with a little more salt and pepper (about 1/2 tsp each), add a parmesan rind (optional), and add 1/2 cup heavy cream.
Cook for 20 minutes on medium to heat the soup through and let the flavors marry together. Taste for seasoning, add more salt or pepper if necessary (and remove parmesan rind, if used). Top with fresh basil (optional). Enjoy!