Preheat oven 250 degrees F. Cut tomatoes in half lengthwise. Gently squeeze out the seeds and place cut-side-up on a non-stick or parchment paper-lined baking sheet. Top each tomato half with a few garlic slices. Drizzle with the olive oil, and then sprinkle with salt and pepper. Roast 1 hour. Remove and place in a soup pot with the stock and potatoes. Simmer 20 minutes, or until potatoes are tender. Puree the soup in a food processor or blender, and then return to the pot. Bring back to a simmer, and then swirl in the basil and season to taste.