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1
Preheat oven to 375u00b0. 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.
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2
Cook leeks in 4-qt. boiling water in a large Dutch oven 5 minutes. Remove leeks with a slotted spoon, reserving boiling water in Dutch oven. Plunge leeks into ice water to stop the cooking process; drain.
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3
Add pasta to boiling water, and cook 10 minutes or until al dente. Drain.
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4
Melt 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, 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.
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5
Gently stir together pasta, cheese sauce, half each of cooked bacon and leeks, and egg yolk. Stir in Gruyere cheese and remaining 1/2 cup 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.
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6
Bake at 375u00b0 for 35 minutes or until golden and bubbly. Let stand 15 minutes before serving.
