Preheat oven to 400 degrees.
Slice about 1/4" from the head of garlic and drizzle olive oil over the exposed cloves. Wrap garlic in aluminum foil and roast for 40-50 minutes, until soft. Remove from oven and cool until you can handle it.
Meanwhile, set a vegetable steamer into a pot or saucepan with a lid. Add an inch of water to the pot and fill the steamer basket with the cauliflower florets. Cover the pot and bring the water to a boil. Cook cauliflower until very tender (so that when you pierce it with a knife it goes in and comes out easily without catching), about 10 minutes.
Transfer cauliflower to a food processor. Squeeze the individual cloves of roasted garlic into the cauliflower. Add the thyme and secure the lid on the food processor. Pulse the cauliflower mixture until there are no lumps and it's smooth like mashed potatoes. This may take a few minutes. Season with salt and pepper to taste. Serve.