Place a steamer in a large saucepan. Add enough water to reach just below it. Bring to a boil. Add the vegetables, cover, and steam until tender, about 7 minutes. Transfer to a bowl and season with 1/4 teaspoon of the salt. Cover to keep warm.
If necessary, add more water to the pan. Return to a boil. Season the halibut with the remaining salt and the pepper. Place the halibut on the steamer, cover, and cook until it flakes easily and is the same color throughout, about 7 minutes.
Drizzle the halibut and vegetables with the oil. Sprinkle with the herbs (if using) and serve the lemon halves on the side.