Preheat the oven to 400°F (200°C).
Boil salted water. Cook macaroni for 8 minutes. Drain.
Melt 1/4 cup butter in a large saucepan on medium heat.
Stir in flour; cook 3-4 minutes until pale yellow.
Add thyme, cayenne, white pepper; cook for 1 minute.
Whisk in 1 cup milk until smooth. Add remaining milk.
Bring sauce to a simmer. Add salt, Worcestershire sauce, and nutmeg.
Simmer, whisking often, for 8 minutes until thickened.
Turn off the heat. Stir in 2 1/4 cups Cheddar, mustard.
Transfer macaroni to the dish. Pour cheese sauce; mix.
Mix panko, and 1 tbsp melted butter in a small bowl.
Sprinkle mixture over macaroni. Add 3/4 cup Cheddar.
Bake until golden brown, about 20 minutes. Enjoy!