Preheat the oven to 220°C/Gas Mark 7 and line 2 baking trays with foil. Place the vegetables on the baking trays, drizzle with the oil and season with salt. Toss well to coat and spread in a single layer.
Roast for 25 minutes, then stir in the balsamic and garlic. Sprinkle over the California Walnuts and roast for 10 minutes more, or until the thickest parts of the vegetables are tender and the walnuts are toasted.
Toss with the herbs and season with pepper.