Preheat oven to 425F (220C).

Arrange tomatoes (cut-side up) and onion on a large parchment paper lined baking sheet. Drizzle all over with olive oil and sprinkle with kosher salt and pepper. Transfer to 425F (220C) preheated oven and bake until tomatoes are softened, juicy, and beginning to turn golden (about 45-60 minutes) (onions should be soft and fork-tender as well). Set aside as you cook the garlic.

In a large nonreactive pot (see note) or Dutch oven, melt butter over medium heat. Add garlic and cook until fragrant.

Carefully add cooked tomatoes and onion and their juices to the pot. Add chicken broth and basil and stir well.

Use an immersion blender to puree the soup until smooth and uniform (alternatively puree the soup in batches in a blender).

Add thyme and heavy cream and stir. Simmer soup, stirring occasionally, for at least 30 minutes.

Remove thyme sprig. Taste-test soup, add additional salt and pepper if needed and if desired stir in baking soda (see note). Serve by itself or topped with croutons, parmesan cheese, a drizzle of olive oil or cream, etc. Oh, or with a cheesy, melty grilled cheese for dipping!