Preheat oven to 425 degrees F. Halve or quarter tomatoes if you are using large ones (you can leave cherry tomatoes whole). Spread them out on a parchment-lined baking sheet along with the sliced onion and the 6 smashed cloves of garlic. Drizzle with the olive oil (2 tablespoons) and season with kosher salt (½ teaspoon) and pepper (¼ teaspoon). Bake at 425 degrees F for 30 minutes (see notes).
Meanwhile, heat up chicken stock (2 cups) in a large pot on the stove.
Add roasted tomatoes/onions/garlic to the warm chicken stock, along with the entire jar of roasted red peppers. Use an immersion blender to puree the soup directly in the pot (or a standing blender in batches).
OPTIONAL: Use a mesh sieve to strain the soup into another pot or large container, using a wooden spoon to squeeze as much liquid out as possible. Discard solids left behind. This will result in a very smooth, bisque-like soup.
Stir in heavy cream (¼ cup). Taste and season with salt and pepper, if necessary. Serve hot, garnished with croutons, fresh basil, more cream, and black pepper, if desired.
