Bring a large pot of salted water to a boil. Cook the shell pasta until al dente according to package directions. Drain and set aside.
Preheat the oven to 400°F
In a large skillet or Dutch oven, melt the 5 tbsp of butter over medium heat.
Whisk in the 1/3 cup of flour and cook for 1 to 2 minutes, stirring constantly, until a smooth paste forms.
Pour in the warm milk and whisk. Continue stirring with a whisk or silicone spatula on low heat until the sauce is smooth and thickened, this can take a few minutes.
Stir in the Dijon mustard, nutmeg, and salt. Turn off the heat.
Add the shredded cheese a handful at a time, stirring until fully melted and smooth. Taste and adjust seasoning if needed.
Add the cooked pasta and stir until evenly coated in the cheese sauce.
Top with the mozzarella cheese and breadcrumbs.
Bake for 15 minutes, or until bubbly and golden brown on top. Serve warm.