Preheat oven to 400°F.
On a large sheet pan, combine: Tomatoes, Red pepper, 1 onion, quartered, Halved garlic heads, Thyme sprigs.
Drizzle with olive oil, season generously with salt and pepper, and add red pepper flakes if using. Toss well.
Roast for 25–30 minutes, until softened, slightly charred, and very fragrant.
In a large pot or Dutch oven, heat a drizzle of olive oil over medium-high heat.
Add: Remaining diced onion, Carrots, Celery, Basil (whole sprigs).
Cook for about 5–7 minutes, until softened and aromatic.
Add the roasted vegetables (including all juices) into the pot.
Stir in: Tomato paste.
Squeeze the roasted garlic out of the skins and into the soup. Mix well.
Let cook for 5 minutes to deepen flavor.
Pour in 1 cup chicken stock, bring to a simmer, cover, and cook on low for 25-30 minutes.
Remove basil stems and thyme stems.
Use an immersion blender (or transfer carefully to a blender) and blend until completely smooth.
Stir in: ½ to ⅔ cup heavy cream (adjust to your preference), 1–2 teaspoons sherry vinegar (this brightens everything up).
Season with salt and pepper to taste.
