- Preheat oven to 350 degrees. Remove papery skin from garlic, leaving heads intact. Place garlic on a sheet of heavy-duty foil; drizzle with 1/4 teaspoon olive oil and sprinkle with a pinch of salt and pepper.
- Loosely wrap foil around garlic, folding foil edges securely. Roast until garlic has softened, about 40 minutes, then transfer to plate. Open carefully and discard foil; let garlic cool.
- Separate garlic into cloves. Squeeze soft garlic from each clove into a small bowl; set aside.
- In a large saucepan over medium heat, heat remaining 1/4 teaspoon olive oil. Add onion, celery, and roasted garlic. Cover and cook until vegetables soften, about 3 minutes. Stir in tomatoes, 1 cup water, bay leaf, basil, oregano, thyme, and 1/4 teaspoon pepper; bring to a boil. Reduce heat and simmer 15 minutes to blend flavors.
- Remove bay leaf. In a blender, puree soup in batches until smooth.