01 - Preheat the oven to 390°F (200°C).
02 - Arrange tomatoes, red onion, garlic, and red bell pepper in a large baking dish. Drizzle with 2 tablespoons olive oil, sprinkle with dried oregano, salt, pepper, and optional red pepper flakes, then toss to coat evenly.
03 - Place the block of feta cheese in the center of the vegetables and drizzle with the remaining tablespoon of olive oil.
04 - Roast in the oven for 30 minutes until vegetables soften and begin to caramelize and feta turns golden.
05 - Carefully transfer the roasted vegetables and feta to a large saucepan using tongs.
06 - Add vegetable broth and bring to a gentle simmer over medium heat for 5 minutes to meld flavors.
07 - Use an immersion blender or standard blender in batches to purée the soup until smooth and creamy, then adjust seasoning as necessary.
08 - Ladle into bowls, garnish with fresh basil leaves, and serve with crusty bread if desired.