For the berries:
To make the roasted berries, preheat the oven to 400˚ F. Line a baking sheet with aluminum foil. Spread the berries in an even layer on the baking sheet. Drizzle with olive oil, add the sugar and season lightly with salt and pepper. Toss gently to coat, then spread again into an even layer. Bake until slightly softened and fragrant, about 10 minutes. Remove from oven and let cool slightly.
Add baby spinach leaves to serving plates. Top with pieces of brie, a sprinkle of toasted nuts, and some of the roasted berries. Lightly dress the salads with vinaigrette of your choice. Serve immediately.