How to Make It
In a medium saucepan of boiling salted water, cook the whole cauliflower florets until almost tender, about 6 minutes; drain. Add the cream and milk to the parboiled florets and simmer over low heat until very tender, about 9 minutes. Strain, reserving the cooking liquid, and puree in a blender until smooth, adding enough cooking liquid to make a loose puree. Stir in the crème fraîche, season with salt and pepper and keep warm.
Preheat the broiler. In a skillet, cook the bacon until lightly crisp; drain on paper towels. Discard the fat and heat the oil. Add the sliced cauliflower and cook over low heat until almost tender and lightly browned on both sides, about 11 minutes. Spread half of the cauliflower slices in a shallow baking dish, cover with the bacon and top with the remaining cauliflower. Add the stock and sprinkle with the Gruyère and Parmesan. Broil for 2 minutes, or until bubbling. Serve the gratin with the puree.