Preheat the oven to Gas Mark 8, 220°C (425°F).
To cook the potatoes: Place the potatoes in a large saucepan, add cold water to cover and season with salt. Bring to a boil, reduce the heat and simmer until tender, approximately 20 minutes. Drain and allow to cool for a couple of minutes.
To cook the fish: Combine the garlic, oil, zest and juice of half a lemon and the thyme leaves. Place the potatoes and fish, skin side down, in a single layer on a large roasting tray. With a potato masher press the potatoes to flatten slightly. Add the broccoli. It can overlap the potatoes. Spoon the garlic, oil and thyme mixture over the potatoes, fish and broccoli. Season with salt and black pepper. Place in the preheated oven and roast for 15 minutes.
To make the dressing: Combine all the ingredients in a small bowl. Taste and add more seasoning if necessary.
To serve: Serve the haddock with the potatoes, broccoli and spoonful of creamy dressing.