Cook pasta shells in a large pot of salted boiling water according to package directions. Drain.
Meanwhile, in a small skillet, melt 2 tablespoons of butter over medium-high heat. Add the breadcrumbs; cook, stirring constantly, until toasted and golden brown, about 4 minutes. Remove from the heat and set aside to use later.
In a large skillet or Dutch oven, melt remaining ¼ cup of butter over medium-high heat. Add flour; cook, stirring until the mixture is golden brown, about 2 minutes.
Gradually whisk in the milk, salt and pepper. Reduce heat to medium; whisk continually until the mixture bubbles and thickens slightly, about 4-5 minutes.
Remove from the heat; gradually whisk in the cheddar, Velveeta, Parmesan, sour cream, ground mustard, garlic powder and nutmeg until the cheese is melted and smooth. Taste and season with additional salt and pepper, if desired.
Add cooked pasta; stir to coat. Sprinkle with toasted breadcrumbs, garnish with fresh herbs, and serve immediately.