- Preheat an oven to 425ºF. Butter a 2-quart baking dish.
- Bring a large pot of water to a boil over high heat. Add the salt and pasta to the boiling water and cook, stirring occasionally to prevent sticking, until not quite al dente, about 2 minutes less than the package instructions. Drain and rinse under cold running water. Set aside.
- In the same pot over medium heat, melt the butter. Add the flour and cook, stirring constantly, for 30 seconds. Gradually add the milk, whisking constantly to prevent lumps from forming, and cook until the sauce is thick and creamy, 5 to 10 minutes.
- Stir in the nutmeg, cheese, peas and prosciutto. Return the pasta to the sauce and toss to combine.
- Transfer the pasta and sauce to the prepared dish. Sprinkle with the bread crumbs. Bake until the bread crumbs are golden and the sauce is bubbling, about 20 minutes.
- Let cool for 10 minutes before serving.