Preheat your oven to 350 degrees Fahrenheit.

In a medium skillet, melt 2 tablespoons of unsalted butter over medium heat. Add the panko and cook while stirring until golden brown, about 6 to 8 minutes. Transfer to a bowl and toss with grated Parmesan, fresh thyme, and a quarter teaspoon of salt. Set aside.

Bring a large pot of salted water to a boil. Cook the cavatappi according to package instructions until al dente. Drain and set aside.

In a separate saucepan, heat 2 1/2 cups of whole milk over medium heat until just simmering.

In another medium saucepan, melt the remaining 2 tablespoons of unsalted butter. Add grated onion and garlic, cooking until fragrant (2 minutes).

Sprinkle in 2 tablespoons of all-purpose flour into the onion mixture, cooking while stirring for about 1 minute.

Gradually whisk in the warm milk until smooth. Bring to a gentle boil and then reduce heat, allowing it to simmer and thicken for 6 to 8 minutes.

Stir in the grated Fontina, Gruyere, and sharp white cheddar until melted. Add the mustard powder and cayenne pepper.

Mix in the cooked cavatappi until fully coated with the cheese sauce.

Pour the mac and cheese mixture into a baking dish, spreading it out evenly.

Bake for about 10 minutes. Sprinkle the toasted breadcrumb mixture on top and return to the oven for another 8 to 10 minutes until bubbly and golden brown.

Let the mac and cheese cool for about 15 minutes before serving.