Preheat oven to 425 degrees F.
In a large oven safe pot, combine tomatoes, garlic, onion, thyme, 5 sage leaves, olive oil, red pepper flakes, and a pinch each of salt and pepper. Transfer to the oven and roast 20-30 minutes, or until the tomatoes begin to blister and release their juices. Let cool slightly.
Meanwhile, make the crostini. Place the baguette on a baking sheet, and rub with olive oil. Transfer to the oven and cook 5 minutes. Flip the bread, sprinkle with sage, and add a slice of brie to each. Return to the oven 5 more minutes, or until the brie is melted.
Transfer the roasted tomato mix and their juices to a blender, puree until smooth. Return the soup to the pot and place over medium heat on the stove. Stir in the milk and cheddar cheese. Cook 5-10 minutes, until warmed. Stir in the basil and season the soup with salt and pepper.
To serve, ladle the soup among bowls, and top with fresh basil. Serve alongside the brie crostini. Enjoy!