For vegetables, arrange oven racks to accommodate 2 baking sheets and preheat oven to 425 F.
Line vegetables up on 2 large rimmed baking sheets, coat with oil, and season with salt. Roast trays for 15 minutes. Rotate top tray to bottom and bottom to top and roast for another 10 to 15 minutes, or until vegetables are tender.
After vegetables are out of oven, add trout to parchment-lined baking sheet, season with salt and pepper, and roast for 8 to 12 minutes, or until cooked to desired doneness.
For Green Goddess Cream, in blender or food processor, combine all ingredients in order listed. Blend until smooth and pale green. Transfer to serving bowl.
Briefly reheat vegetables in oven if desired, though room temperature or chilled is a really nice way to eat this dish, especially at lunch. Arrange roasted vegetables on plates and add large flaked pieces of trout. Top vegetables and trout with a drizzle of Green Goddess Cream and sprinkle of capers. Serve with additional dressing, if desired.