Preheat the oven to 425 degrees F. In a large oven safe pot, combine the tomatoes, onion, garlic, thyme, 5 sage leaves, olive oil, red pepper flakes, and a pinch each of salt and pepper. Transfer to the oven and roast for 20-30 minutes or until the tomatoes begin to char and release their juices. Let cool slightly.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. Meanwhile, make the crostini. Finely chop the remaining 3 sage leaves. Place the bread 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 and cook 5 more minutes or until the brie is melted. To serve, ladle the soup among bowls and top with fresh basil. Serve alongside the brie crostini. Enjoy!