Cauliflower Steaks with Cauliflower Purée

Preheat oven to 250°F. Using sharp heavy knife and starting at top center of cauliflower head, cut two 1-inch-thick slices of cauliflower, cutting through stem end. Set cauliflower steaks aside.
Cut enough florets from remaining cauliflower head to measure 3 cups. Combine florets, 1 1/2 cups water, and milk in medium saucepan, and sprinkle with salt and pepper. Bring to boil and cook until cauliflower florets are very tender, about 10 minutes. Strain, reserving 1 cup cooking liquid. Spread florets on large rimmed baking sheet, and bake 10 minutes until slightly dry. Transfer florets to blender. Add reserved 1 cup cooking liquid and puree until smooth. Return puree to same saucepan and increase oven temperature to 350°F.
Heat 2 tablespoons vegetable oil in heavy large ovenproof skillet over medium-high heat. Brush cauliflower steaks with additional oil and sprinkle with salt and pepper. Add cauliflower steaks to skillet and cook until golden brown, about 2 minutes per side. Transfer skillet to oven and bake cauliflower steaks until tender, about 10 minutes.
Rewarm cauliflower puree over medium heat. Divide puree between 2 plates; top each with cauliflower steak.