Preheat the oven to 425°F.

Prepare the vegetables: Toss potatoes and Romanesco florets with 3 tablespoons olive oil, 2 thyme sprigs, 1 garlic clove, salt, and pepper on a rimmed baking sheet.

Roast the vegetables for 20 minutes, until tender and browned.

Add olives to the vegetables and roast for an additional 3 minutes, until warmed.

Discard the thyme sprigs from the vegetables.

Prepare the fish: Heat 2 tablespoons olive oil in a large skillet over medium heat.

Season halibut fillets with salt and pepper.

Add the fish to the skillet, skin-side down, with 1 thyme sprig and 1 garlic clove.

Cook for about 10 minutes, until the skin is golden brown and the fish releases easily from the pan.

Turn the fish, add 2 tablespoons more olive oil to the skillet, and cook, spooning oil over the fish, for about 3 minutes, until the fillets are white throughout.

Transfer the cooked fish to plates.

Prepare the salad: Transfer roasted vegetables to a large bowl and toss with lemon juice, fish sauce, parsley, and 2 tablespoons olive oil.

Season the salad with salt and pepper to taste.

Serve: Spoon the roasted vegetable salad around the fish fillets and serve immediately.