Grate 1/2 of the Cheddar cheese; cut the rest into small cubes.
Place bacon in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned and crispy, about 10 minutes. Drain bacon slices on paper towels.
Preheat oven to 375 degrees F (190 degrees C).
Bring a large pot of lightly salted water to a boil. Cook elbow macaroni in the boiling water, stirring occasionally, until tender yet firm to the bite, about 8 minutes. Drain.
Melt butter in a saucepan over medium-low heat. Add flour; cook and stir for 1 minute. Stir in milk gradually; cook until sauce thickens, about 6 minutes. Add grated Cheddar cheese and Dijon mustard. Season with salt and pepper. Stir cubed Cheddar cheese and leek rings into the sauce.
Pour sauce into the pot of macaroni; stir well. Pour into a baking pan.
Bake in the preheated oven until bubbling and golden, about 20 minutes. Sprinkle bacon on top.
