Preheat oven to 350° F.
Core the tomatoes and slice in half. Add the tomatoes, quartered red onion, and whole garlic cloves to a large bowl. Drizzle with ⅓ cup of olive oil, toss to coat. Spread the mixture on a large baking sheet with the tomatoes cut side up. Sprinkle the vegetables with sea salt and bake for 30-40 minutes or until the vegetables are soft and slightly caramelized.
In the carafe of a blender add ¼ cup basil leaves, crushed red pepper, sugar, and tomato paste.
When the tomatoes are roasted add them to the blender along with the roasted onion. Squeeze the garlic cloves into the blender discarding the skin. Pour any pan juices into a large soup pot and set aside.
Blend the tomato and onion mixture until smooth and creamy.
Pour the puree into the soup pot and add 3 cups of vegetable stock. Add up to another cup of vegetable stock or until you reach the desired consistency.
Stir in the fresh thyme, salt and pepper to taste. Heat on medium until hot - do not boil.
Serve garnished with a drizzle of olive oil, basil and grated Parmesan if desired.