Preheat your oven to 400°F and line a large baking sheet with parchment paper.
Slice the Roma tomatoes in half and place them on the baking sheet along with the peeled garlic cloves.
Drizzle the tomatoes and garlic with half of the olive oil, then sprinkle with the sea salt, black pepper, and dried oregano.
Roast for 25 minutes until the tomatoes are soft, blistered, and slightly caramelized.
Transfer the roasted tomatoes and garlic into a high-speed blender, adding the chicken bone broth, cooked chicken breast, and fresh basil.
Blend on high until the mixture is completely smooth and reaches a velvety consistency.
While the soup is blending, toast the sourdough slice until golden, drizzle with the remaining olive oil, and top with the parmesan cheese.
Place the crostini under the broiler for 1 minute until the cheese is melted and bubbly.
Pour the hot soup into a bowl and serve immediately with the crunchy parmesan crostini on the side.