Preheat Oven: Arrange oven rack on middle rack and preheat oven to 425 degrees F.
Bring Chicken Stock to a Boil: In a large pot over medium-high heat, bring chicken stock to a boil.
Prepare and Simmer Cauliflower: Meanwhile, using a knife, cut off bottom of cauliflower and remove any outer leaves. Do not break cauliflower apart. Transfer cauliflower to pot of stock and simmer uncovered for 15 minutes.
Prepare Cauliflower to Roast: Using tongs, transfer cauliflower from pot to 8x8-inch baking pan or large cast-iron skillet. Spoon some chicken stock atop cauliflower, drizzle on melted butter, then sprinkle on grated Parmesan, thyme, salt, and pepper.
Roast Cauliflower: Transfer baking pan to the oven and bake until cauliflower is golden, about 9-10 minutes, basting with cooking juice halfway through. Then, move pan to highest oven rack, turn on oven broiler, and broil for additional 30-60 seconds.
Serve: Remove pan from oven, drizzle on cooking juice, then serve.