Make the béchamel:

Combine milk, butter and flour in a heavy medium saucepan; bring to a boil, then reduce to a simmer and cook, stirring constantly, first with a whisk, then with a wooden spoon, until sauce is very thick, about 20 minutes. Season to taste with salt, pepper and nutmeg. Remove béchamel from heat, strain through a fine-mesh sieve into a small bowl, and cover surface with plastic wrap to keep skin from forming.

Cook the cauliflower:

Remove any leaves from cauliflower, leaving core intact. Cut cauliflower into quarters and put in a large pot. Add water to come up 1 inch. Cook, covered, over high heat until cauliflower is tender, about 20 minutes; then drain.

Trim tough stems from cauliflower, then, working with 1 quarter at a time, wrap cauliflower in a large, clean dishtowel (do not use a new, unwashed, colored dishtowel) and, over sink, twist dishtowel until all excess liquid is removed from cauliflower. Pulse cauliflower in a food processor until finely chopped.

Make the custard:

Heat oven to 325° F. Beat eggs in a large bowl; add béchamel and whisk to combine. Add cauliflower, cheese, salt and pepper; stir to combine.

Butter a 1 1 /2- to 2-quart baking dish (or a similar size mold) with 2 tablespoons butter (it is important to use all of the butter) and coat with bread crumbs. Add cauliflower mixture. Bake until sformato sets and lightly browns, about 40 minutes. Remove the sformato from oven; let rest for 15 to 20 minutes, then unmold (optional). Serve warm or at room temperature.