How to Make It
Preheat the oven to 425°. On a rimmed baking sheet, toss the potatoes and Romanesco with 3 tablespoons of the olive oil, 2 thyme sprigs, 1 garlic clove and a generous pinch each of salt and pepper. Spread the vegetables in an even layer and roast for 20 minutes, until tender and browned on the bottom. Scatter the olives on top and roast until warmed, about 3 minutes longer. Discard the thyme.
Meanwhile, in a large skillet, heat 2 tablespoons of the olive oil. Generously season the fish with salt and pepper and add to the skillet, skin side down, along with the remaining thyme sprig and garlic clove. Cook over moderate heat until the fish is golden brown on the bottom and releases easily from the pan, about 10 minutes. Turn the fish, add 2 more tablespoons of olive oil to the skillet and cook, spooning the oil over the fish, until the fillets are white throughout, about 3 minutes. Transfer the fish to plates.
Transfer the roasted vegetables to a large bowl and toss with the lemon juice, fish sauce, parsley and the remaining 2 tablespoons of olive oil. Season with salt and pepper, spoon around the fish and serve.