Cook pasta according to package directions without added fat or salt; drain and transfer to a large bowl. While pasta is still hot, stir in sour cream; set aside.
Heat milk in a small saucepan over medium heat until tiny bubbles appear just around the edges (known as scalding). Reduce heat to low, add cheese and simmer until cheese melts, stirring constantly with a wire whisk, about 2 minutes; remove from heat and stir in mustard, salt, pepper and nutmeg.
Add cheese mixture to pasta; mix well. Transfer to a 3-quart casserole dish.
Combine bread crumbs and Parmesan cheese; sprinkle over pasta.
Bake until top is golden, about 30 minutes. Yields about 1 cup per serving