Preheat oven to 450 degrees.
Place the tomatoes, garlic, and onions on a large rimmed baking sheet. Add the olive oil and generously season with salt and pepper. Toss to combine.
Roast in the oven for 30 to 35 minutes, or until tomatoes are lightly browned and caramelized. Carefully transfer roasted vegetables to a large pot and add the vegetable broth. Over medium-high heat, bring to a boil. Lower the heat to medium and simmer for 10 minutes.
Remove from heat and add the 1/2 cup of the freshly chopped basil. Using an immersion blender, carefully blend until smooth. Alternatively, you can transfer the soup in small batches to a blender, and blend until smooth. If using a blender, the blender should never be more than half full since the heat will cause the mixture to expand.
If using a blender, return soup to the pot. Add the brown sugar and lemon juice. Add additional salt and pepper to taste.
Serve the bisque with croutons and additional chopped basil.