Center a rack in the oven and preheat to 425°F. Line a baking sheet with parchment paper. Generously butter an oven-safe pan that holds about 2½ quarts. Place the dish on the baking sheet.
Put a large pot of salted water on to boil. Pull or cut the florets from the cauliflower, leaving about an inch or so of stem. Drop the florets into the boiling water and cook for 10 minutes. Drain and rinse the cauliflower under cold running water to cool it down, and pat it dry. (Alternatively, you can steam the florets. When they’re fork-tender drain and pat dry.)
While the cauliflower is cooking, toss the bacon strips into a heavy skillet. Put the skillet over medium heat and cook until the bacon is browned, but not crisp. Drain and pat dry.
Spread the cauliflower in the buttered pan and scatter bacon bits over the top.
Put the flour in a bowl and gradually whisk in the eggs. When the flour and eggs are blended, whisk in the cream and milk. Season the mixture with salt, pepper, and nutmeg, and stir in about two-thirds of the cheese. Pour the mixture over the cauliflower, shake the pan a little, so that the liquid settles between the florets, and scatter over the remaining cheese.
Bake the gratin for about 25 minutes, or until it is puffed and golden and a knife inserted into the center comes out clean. If the top isn’t as brown as you’d like it to be, put it under the broiler for a couple of minutes.