Arrange cauliflower tightly into a 9x13-inch glass baking dish. Pour melted butter over cauliflower and lightly season with salt and pepper. Place garlic cloves into the cauliflower.
Roast in the preheated oven until cauliflower is tender and browned, about 30 minutes.
Heat olive oil in a large skillet over medium heat. Cook and stir pancetta in the hot oil until browned and crisp, 5 to 10 minutes. Transfer pancetta to paper towel-lined bowl, reserving pancetta drippings in the skillet.
Cook and stir onion in the pancetta drippings until tender, about 5 minutes. Mix thyme, rosemary, and turmeric into onion.
Pour chicken stock over onion mixture and bring to a boil. Add cauliflower and garlic to chicken stock. Reduce heat to medium and simmer soup until cauliflower is very soft, about 15 minutes. Stir pancetta into soup.
Blend soup using an immersion blender until completely smooth. Stir Cheddar cheese, 1 to 2 tablespoons at a time, into soup until melted.