A recipe from Hugh Acheson of the Empire State South restaurant in Atlanta. A Canadian that moved south. So maybe this is Canadian Southern Mac and Cheese! Posted as a side, but could also be a main dish.
Preheat oven to 375°. Cook bacon in a skillet over medium heat, stirring occasionally, 6 to 8 minutes or until crisp; remove bacon, and drain on paper towels. Discard drippings.
Cook leeks in 4 quarts of boiling water in a large Dutch oven for 5 minutes. Remove leeks with a slotted spoon, reserving boiling water in the Dutch oven. Plunge leeks into ice water to stop the cooking process. Drain.
Add pasta to the boiling water; and cook 1 minutes or until al dente. Drain.
Melt the butter in a large skillet over medium heat. Reduce heat to medium-low, and whisk in flour until smooth; cook, whisking constantly 2 minutes or until golden brown. Slowly whisk in milk, and cook, whisking constantly for 3 minutes or until thickened. Whisk in dry mustard, next 3 ingredients and 1/2 cup Cheddar cheese, stirring until cheese is melted. Remove from heat. Season with salt to taste.
Gently stir together pasta, cheese sauce, half each of cooked bacon and leeks, and egg yolk. Stir in Gruyere cheese and the remaining Cheddar cheese. Spoon pasta mixture into a buttered 8 inch cast iron skillet, and sprinkle with remaining bacon and leeks. Drizzle with cream; sprinkle with breadcrumbs.
Bake at 375° for 35 minutes or until golden and bubbly. Let stand 15 minutes before serving.