Step 1 Preheat the oven to 350°F. Using a sharp heavy knife and starting at top center of cauliflower head, cut two 1-inch-thick slices of cauliflower, cutting through stem end and leaving the leaves intact. Set the cauliflower steaks aside. Cut the remaining florets into ½-inch pieces.
Step 2 In a medium pot, bring the chicken or vegetable broth and ½ teaspoon salt to a boil. Add the cauliflower and bring back to a boil. Cover, reduce the heat to low and steam for about 15 minutes, or until the cauliflower is very tender. Using a slotted spoon, transfer the cauliflower to a food processor. Add ½ cup of the broth from the pot, along with the butter, and process until smooth. Taste and adjust seasoning with salt and freshly ground pepper. Add the thyme and process until just combined. Discard the remaining broth and transfer the cauliflower purée back to the pot.
Step 3 Heat 1½ tablespoons of the vegetable oil in a large ovenproof skillet over medium-high heat. Add the cauliflower steaks to the skillet, nice side down (whichever side is prettier), and then drizzle the cauliflower steaks with the remaining ½ tablespoon of oil. Sprinkle each steak with ¼ teaspoon salt and a few grinds of fresh black pepper. Cook until the first side is golden brown, 4-5 minutes. Flip the cauliflower steaks over and transfer the pan to the oven. Bake until tender, about 10 minutes.
Step 4 Rewarm the cauliflower purée over medium heat. Divide the purée between 2 plates, and then top each with a cauliflower steak, nice side up.
