Preheat the oven to 375°F. Butter and 8-inch square (1 1/2 quart) baking dish or pan (or 6 8-ounce ramekins). Set aside.

Fill a 4- to 5-quart pot about three-quarters full and add 1 tablespoon of the salt. Bring to a boil and add the pasta and sun-dried tomatoes. Cook, stirring once or twice, until tender but firm, 8 to 10 minutes, and drain. Reserve the pot.

Heat a medium skillet over medium-low heat. Add the pine nuts and cook, shaking the pan constantly, until they are a light golden color. Watch carefully, because they burn easily. Remove from the heat and transfer to a small bowl. Using the same skillet, melt 2 tablespoons of the butter over medium heat. Turn off the heat and add the bread crumbs, Parmigiano-Reggiano, and pine nuts. Stir until mixed well. Set aside.

Using the same pot you used to cook the pasta, melt the remaining 2 tablespoons butter over medium heat. Slowly whisk in the flour and stir constantly until a paste forms, 30 to 45 seconds. Continue stirring for 1 to 2 minutes more, until the mixture starts to darken slightly and smell a bit nutty. Slowly whisk in the milk, cream, and the remaining 1 teaspoon salt and cook until the mixture starts to thicken and is just beginning to bubble around the edges, 5 to 7 minutes. It should be thick enough to coat the back of a wooden spoon. Add the Havarti, Taleggio, rosemary, mustard powder, cayenne, and nutmeg and stir until the sauce is smooth but not too runny. It should be similar in texture to cake batter. If it's soupy, continue cooking until it thickens.

Add the pasta and stir to combine. Pour into the prepared baking dish. Top with the bread crumb mixture and scatter the prosciutto strips over the bread crumbs. Place the dish on a rimmed baking sheet and bake until the prosciutto is crisp and the mac & cheese is bubbling and golden brown, about 30 minutes. Let cool for 10 to 15 minutes before serving.