Arrange oven rack on middle rack and preheat oven to 425 degrees. In a large pot over medium-high heat, bring chicken stock to a boil.
Meanwhile, using a knife, cut off bottom of cauliflower and remove any leaves. Do not break cauliflower apart.
Transfer cauliflower to pot of stock and simmer uncovered for 15 minutes.
Using tongs, transfer cauliflower from pot to 8×8-inch baking pan. Spoon some chicken stock atop cauliflower, drizzle on melted butter, then sprinkle on grated Parmesan, thyme, salt, and pepper.
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 seconds – 1 minute.
Remove pan from oven, drizzle on cooking juice, and serve.