Serve this soup with slices of crusty bread, slathered with the roasted garlic and tomatoes.
Heat oven to 450 degrees with two racks centered. Spray two 12-by-14-inch roasting or jelly-roll pans with cooking spray. Arrange potatoes in one pan and tomatoes in second pan. Add garlic to one of the pans. Drizzle potatoes with oil; toss. Sprinkle potatoes with 1/2 teaspoon salt and 1/4 teaspoon pepper. Sprinkle tomatoes with remaining 1/2 teaspoon salt and 1/4 teaspoon pepper. Sprinkle one teaspoon of savory or thyme over ingredients in each pan. Roast 20 minutes; turn potatoes and tomatoes with a spatula. Sprinkle curry powder over both pans. Cook until potatoes and garlic are tender, 20 to 30 minutes more.
Transfer potatoes to a food mill fitted with a medium disk, in batches if necessary. Pass potatoes through mill; transfer to a stockpot. Add 2 cups stock to roasting pan; place over high heat on stove. Cook, scraping up any cooked-on bits with a wooden spoon, until liquid reduces slightly 2 to 4 minutes. Add liquid to stockpot; stir until smooth.
Using back of a knife, scrape soft, roasted garlic out of 4 to 5 cloves; reserve. Scrape out remaining garlic; set aside. Set aside 12 pieces of tomato for garnish. Transfer tomatoes to food mill with the 4 to 5 cloves of garlic; pass through into a large bowl. Transfer to stockpot. Add remaining 1 1/2 cups stock and milk. Place over medium heat; stir until warm and combined. Adjust seasoning. Just before serving, add lemon juice to taste.
Toast bread slices under the broiler. In a small bowl, smash reserved garlic cloves with a fork until smooth. Chop leaves from two sprigs of thyme. Slather toast with garlic puree, top with reserved tomatoes, and sprinkle with chopped thyme. Use remaining thyme to garnish soup.
The head of garlic is roasted to mellow its flavor.