Bring a pot of salted water to a boil. Add the pasta and cook as the label directs. Reserve 3/4 cup of the cooking water, then drain the pasta.

Add the butter to the empty pot and melt over medium-high heat. Add the shallot and cook, stirring, until slightly softened, about 2 minutes. Add the flour and mustard powder and cook, stirring, until the flour begins to toast, about 1 minute. Whisk in the milk and the reserved cooking water; cook, whisking, until thick, about 2 minutes.

Reduce the heat to medium. Whisk in the sour cream, mozzarella, Swiss cheese and 1 tablespoon parmesan; continue whisking until melted, about 1 minute. Stir in the pasta, 1/2 teaspoon salt and 1/4 teaspoon pepper; transfer to a 2-quart baking dish. Preheat the broiler.

Combine the remaining 2 tablespoons parmesan, the herbs, and salt and pepper to taste in a small bowl. Sprinkle over the pasta. Broil until golden and bubbly, about 2 minutes.