How to Make It
Preheat the oven to 375°. Grease a 3-quart baking dish with butter.
In a large pot of salted boiling water, cook the cauliflower florets until crisp-tender, about 3 minutes. Using a slotted spoon, transfer the cauliflower to the prepared baking dish and spread in an even layer. Add the macaroni to the boiling water and cook until al dente. Drain well.
Meanwhile, in a large saucepan, melt the 2 tablespoons of butter over moderate heat. Add the flour and cook, stirring, for 2 minutes. Gradually stir in the milk until smooth. Bring the sauce to a boil over moderately high heat, then reduce the heat to moderate and cook, stirring occasionally, until thickened, about 15 minutes.
Remove the pan from the heat and stir in the ricotta and nutmeg. Stir in the pasta, then the Gruyère, and season with salt and pepper. Spread the pasta and cheese over the cauliflower and bake for 20 minutes. Remove the baking dish from the oven and preheat the broiler. Broil 8 inches from the heat until the top browns, about 4 minutes, then serve.