Preheat your oven to 400°F (200°C). Arrange the sliced fennel, halved tomatoes, and sliced onion on a baking sheet. Drizzle with a tablespoon of olive oil and season with a pinch of salt. Roast until vegetables are fragrant, caramelized, and slightly charred around the edges, about 30-35 minutes.
While the vegetables roast, heat a tablespoon of olive oil in a large pot over medium heat. Add the minced garlic and cook until fragrant, about 30 seconds, until it begins to turn golden.
Add the roasted fennel, tomatoes, and onion to the pot. Pour in the vegetable broth and bring the mixture to a gentle simmer. Let it cook uncovered for 10 minutes to allow flavors to meld and the soup to thicken slightly.
Remove the pot from heat and add fresh basil leaves. Using an immersion blender, carefully blend the soup until smooth and velvety. Alternatively, transfer the soup to a blender in batches and blend until smooth.
Taste the soup and season with salt and pepper as needed. Return to the stove on low heat if you wish to warm it further, or serve immediately.
Ladle the bright, vibrant soup into bowls, garnish with a few basil leaves, and enjoy the comforting, aromatic flavors of this seasonal dish.
