To speed things up, boil the water in a kettle. Pour the water into a steaming pot, then lay the vegetables in baskets in layers, starting from one that takes longest to cook : potato, carrot and on highest basket broccoli.
Afterwards season the fish with salt, pepper and lemon juice. Grilling the fish takes only about 5-10 minutes. Therefore you can heat up the grill now, but don't place the fish inside until the vegetables are halfway cooked.
Serve the dish when the sea bass is cooked through and vegetables are also cooked, but not super soft, try to eat them when they still are a bit al dente.
Enjoy!