Preheat oven to 400 degrees. Combine tomatoes, olive oil, salt and pepper. Roast on a lipped baking sheet for about 35-40 minutes, or until edges are lightly browned. Meanwhile, in a large pot, melt butter. Add garlic, onion & celery and cook over medium heat about 10 minutes. Add canned tomatoes with their juice, chicken broth, thyme and the roasted tomatoes, including the juice on the baking pan. Bring the mixture to a boil, then reduce heat and simmer, covered, for about 20 minutes. Allow soup to cool slightly, then use a food processor or blender to puree the soup in batches until smooth. Return to pot and stir in half and half, heating just til warmed through. Top with croutons just before serving.