Place a steamer in a large saucepan. Add enough water to reach just below it. Bring to a boil.
Add the broccoli and squash and steam, covered, until tender, about 7 minutes. Transfer to a bowl, season with ¼ teaspoon salt, and cover.
If necessary, add more water to the pan. Return to a boil. Season the halibut with ½ teaspoon salt and ¼ teaspoon 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 with the lemon halves.