Preheat oven to 400 degrees F (200 degrees C).
Fill a large pot with lightly salted water and bring to a boil over high heat.
Cook the pasta uncovered in the boiling water, stirring occasionally, until the pasta is cooked through but still slightly firm, about 10 minutes.
Melt 1/4 cup butter in a large saucepan over medium heat and when it starts to bubble, add the flour and stir for about 4 minutes.
Add thyme, cayenne pepper, and white pepper; cook and stir another minute, then whisk in 1 cup of milk until smooth.
Pour in remaining milk and whisk again.
Bring the sauce just to a simmer.
Stir in nutmeg, Worcestershire sauce, and salt; simmer on medium-low heat until thickened, about 8 minutes, whisking often.
Turn heat off, then add 2 1/4 cups of Cheddar cheese; stir until melted and combined.
Add Dijon mustard.
Transfer the macaroni into a casserole dish, then pour in the cheese sauce; stir to thoroughly combine sauce with pasta.
Mix panko bread crumbs and 1 tablespoon melted butter in a small bowl, and sprinkle crumbs on top of macaroni and cheese.
Sprinkle remaining 3/4 cup of Cheddar cheese on top.
Bake in the preheated oven until bread crumbs and Cheddar cheese topping are golden brown, about 20 minutes.