1 medium head of cauliflower (1 3/4 pounds), cut into 1-inch florets
1 tablespoon unsalted butter
3 tablespoons extra-virgin olive oil
1 small onion, finely chopped
2 garlic cloves, minced 2 teaspoons minced rosemary
Freshly ground pepper
1/2 cup dry white wine
1 1/2 cups heavy cream
1 1/2 cups freshly grated Parmigiano-Reggiano cheese (6 ounces)
1/2 pound Taleggio cheese—rind discarded, cheese cubed
1 pound large shells, such as conchiglioni
1/4 cup dry bread crumbs
Preheat the oven to 450°. Bring a large pot of salted water to a boil and fill a large bowl with cold water. Boil the cauliflower until tender, 5 minutes. Using a slotted spoon, transfer the cauliflower to the cold water and let cool slightly. Drain and pat dry; keep the cooking water hot.
In a large, deep skillet, melt the butter in the olive oil. Add the onion, garlic and rosemary, season with salt and pepper and cook over moderate heat until softened, about 3 minutes. Add the cauliflower and cook over moderate heat, stirring occasionally, until browned in spots, about 8 minutes. Add the wine and boil until evaporated. Remove the skillet from the heat and stir in the cream, 1 cup of the Parmigiano and the Taleggio.
Return the water to a boil and add the pasta. Cook until al dente. Drain the pasta and return it to the pot. Scrape the cauliflower and cheese sauce into the pasta and toss well. Spread half of the pasta into a 3-quart baking dish and top with 2 tablespoons of the bread crumbs. Top with the remaining pasta, 2 tablespoons of bread crumbs and 1/2 cup of Parmigiano.
Bake the pasta for about 20 minutes, until the sauce is bubbling and the top is golden and crisp. Let stand for 10 minutes before serving.
Make Ahead: The unbaked assembled pasta dish can be refrigerated overnight. Return the pasta to room temperature before baking.