Thinly slice a few cauliflower florets on a mandoline. Toss them in a small bowl with lemon juice and a pinch of salt. Let them marinate at room temperature to soften.
Simmer the majority of the cauliflower florets in milk and a knob of butter until tender. Drain (keep a splash of liquid) and blend with another knob of butter until completely smooth. Add salt as needed. Reserve 2 tablespoons of this purée for the curry sauce.
Clean and slice the leeks. Place several cauliflower florets and the sliced leeks into a small saucepan. Add butter nobs, add fresh thyme, and peppercorns. Cover with water. Cook slowly over low heat until the vegetables are tender. Before serving, halve the florets and quickly pan-roast them for color.
In a small pan, combine curry powder and stock. Simmer to reduce slightly. Whisk in the reserved cauliflower purée to thicken the sauce and add a creamy texture.
Preheat your water bath to 122°F (50°C). Season the halibut with salt and pepper. Place in a vacuum bag with butter and thyme. Submerge and cook for 30 to 45 minutes.
Spread a layer of the remaining cauliflower purée on each plate. Place the halibut fillets on top. Garnish with the confit leeks, roasted cauliflower, and the lemon-marinated shavings. Finish with a generous drizzle of the creamy curry sauce.
