Using a sharp bread knife, slice the head of cauliflower in half. Then from each half, slice off a slab 1 inch thick. Set aside.

Cut the remaining cauliflower into pieces to make about 3 cups of florets. Set aside.

Melt butter in a large saucepan over medium heat. When foamy, add onions, and cook until tender, about 7 minutes. Add milk, water, thyme, bay leaf, about 1/2 teaspoon of salt, pepper to taste, and cauliflower florets.Bring to a boil, then simmer until cauliflower is very tender, about 15 minutes.

Remove bay leaf and drain, reserving 1 cup of the liquid. Place cauliflower florets in a blender with 1/2 cup of reserved liquid. Blend on high until smooth, adding more liquid if needed. Add curry powder and nutmeg and blend to mix. Taste and add salt and pepper as needed. Return to clean saucepan and set aside.

Preheat oven to 350°F.

Heat oil in a large oven-proof skillet over medium-high heat. When hot, add cauliflower slabs, sprinkle with salt and pepper, and sear until golden brown, about 4 minutes per side.

Transfer skillet to oven and bake, flipping cauliflower slabs about halfway through, until tender, 15 to 20 minutes.

To plate, re-warm cauliflower sauce and pour over plates. Place a seared cauliflower slab in the center.