Adjust the oven rack to the center position and preheat the oven to 425° F (232°C). Generously butter a 2 1/2-quart ovenproof dish and place it on a rimmed baking sheet lined with a Silpat or parchment paper. (It’s not elegant and it’s a tad too big, but a 9-by-13-inch baking dish works fine.).
Bring a large pot of salted water to a boil. Cut florets from the cauliflower, leaving about an inch or so of stem. Drop the florets into the boiling water and cook until fork-tender, anywhere from 4 to 10 minutes, depending on the size of the florets. (Alternatively, you can steam the florets over salted water.) Drain and rinse the cauliflower florets under cold running water to cool them and pat them completely dry.
While the cauliflower is cooking, toss the bacon strips into a heavy skillet, place the skillet over medium heat, and cook just until the bacon is browned but not crisp. Drain and pat dry.
Spread the cauliflower in the buttered pan and scatter the bacon over the top.
Place the flour in a bowl and gradually whisk in the eggs until blended. Slowly whisk in the cream and milk. Season the mixture with the salt, pepper, and nutmeg and stir in about 2/3 of the cheese. Pour the mixture over the cauliflower and bacon, shaking the pan a little so that the liquid settles between the florets. Scatter the remaining cheese over the top.
Bake the cauliflower-bacon gratin for about 25 minutes, or until puffed and golden and a knife inserted in the center comes out clean. If the top isn’t as brown as you’d like, run the gratin under the broiler for a couple of minutes.
The gratin is best just from the oven or warm, although it can be enjoyed at room temperature, just like a quiche. You really should eat the cauliflower-bacon gratin the day it’s made, but if you’ve got leftovers, cover and refrigerate them, then let them come to room temperature or warm briefly and gently in the oven.