Croutons: Preheat oven to 325 Fahrenheit. Toss bread cubes with oil, salt, and pepper., then spread on a rimmed baking sheet in one layer, Bake 25 to 35 minutes until crisp and lightly browned, turning with a spatula every 10 minute.
Bisque: Preheat oven to 450 degrees.Spread tomatoes, garlic, and onions on a rimmed baking sheet and drizzle with olive oil. Generously season with salt and pepper, then roast for 30 to 35 minutes until tomatoes are lightly browned and caramelized.
Transfer vegetables to a large pot and add broth. Bring to a boil, then simmer for 10 minutes. Add basil off the heat, then puree by batches in a blender, or puree in the pot with an immersion blender. Blend until it's very smooth, then return it to the pot and add the brown sugar and lemon juice. Taste for seasoning. Serve garnished with croutons and basil chiffonade.