1Preheat oven to 375^ .
In a cast iron skillet fry bacon and onion until bacon is crisp; remove and drain pieces on paper towels. Save drippings for another day.
2Cook pasta according to package direction to the al dente stage, drain.
Melt butter in the skillet over medium heat. Turn down heat to medium low, and whisk in flour until smooth; cook, whisking constantly,about 2 minutes until golden brown. Slowly whisk in 1 cup of the cream, and cook for 3 minutes all the while whisking until thickened. Whisk in mustard salt, red and black pepper. Add 1/2 cup of the Cheddar cheese stirring until cheese is melted. Remove from heat. Add more salt to taste if necessary.
3Gently stir in pasta,1/2 the bacon and onions mixture and egg yolk. Stir in Gruyère cheese and remaining 1/2 cup of Cheddar cheese. Sprinkle remaining 1/2 of bacon and onion mixture over top. Drizzle with remaining cream; sprinkle with Panko and bake for 35 minutes-until golden and bubbly. Let stand 15 minutes before serving.