Preheat oven to 300ºF and line a large rimmed baking sheet with parchment paper or a Silpat™ baking sheet.
Arrange sliced tomatoes on the prepared baking sheet without overcrowding. Sprinkle with garlic powder, and season with salt and black pepper, to taste. Place baking sheet in pre-heated oven and roast approximately 2 hours or until the tomatoes are browned around the edges.
When the tomatoes are almost ready, cut each slice of sourdough bread into equal-sized rectangles and brush one side with melted butter. Place buttered side down on a grill pan set over medium heat. Once grill marks appear, remove from heat and flip each piece of sourdough over so the grill marks are facing up. Set aside.
Remove tomatoes from oven and transfer to a high-powered blender or food-processor, along with the broth, basil, yellow onion, celery, garlic, cumin, and heavy cream. Season with salt and black pepper, to taste.
Blend on high until smooth and thoroughly combined. Taste and adjust seasonings as needed. If desired, add additional broth or water, one tablespoon at a time, and re-blend to achieve desired consistency. Set aside.
Place top oven rack on the second highest position and pre-heat the broiler to high.
Top each slice of grilled bread with a piece of sliced Brie and place the grill pan under broiler for 1-2 minutes, or until the Brie is soft and just starting to melt. Don’t broil too long or the Brie will run off the bread base.
To serve, transfer the hot soup to individual serving bowls and top with the grilled cheese croutons and a sprig of fresh basil, if desired. Enjoy!