Position a rack in the center of the oven and heat the oven to 375°F.
Core the cauliflower and separate it into florets. Cut the florets into 1/4-inch-thick slices. Put the cauliflower on a rimmed baking sheet and toss with the olive oil. Spread in an even layer, season with 1/2 tsp. salt, and roast until tender, 30 to 35 minutes. Let cool.
Raise the oven temperature to 400°F. Brush an 8x10-inch (2-quart) gratin dish with olive oil and evenly coat with about half of the Parmigiano. Set aside.
Heat the milk in a 2-quart saucepan over medium heat until just about to boil.
Meanwhile, melt the butter in a 4-quart saucepan over medium-low heat. Whisk in the flour and cook, whisking constantly, until the mixture turns light golden, 2 to 4 minutes. Slowly add the hot milk, whisking constantly until very smooth. Bring just to a boil and reduce the heat to maintain a simmer. Add 1 tsp. salt and cook, stirring frequently, for 10 minutes to develop the flavor. Transfer to a large bowl.
In a small bowl, whisk the eggs, yolks, and extra-virgin olive oil. Roughly chop 1/2 cup of the cauliflower, and purée the rest in a food processor. Stir the chopped cauliflower, cauliflower purée, egg mixture, and the remaining Parmigiano into the sauce. Season to taste with salt and pepper. Pour the mixture into the prepared gratin dish and bake until the sformato is just set and browned around the edges, about 30 minutes. It should jiggle just a little when you remove it from the oven. Let rest for 10 to 15 minutes before serving.
nutrition information (per serving):
26, Fat Calories
230, Saturated Fat
10, Monounsaturated Fat
16, Polyunsaturated Fat
Photo: Scott Phillips