Preheat the oven to 350°F (175°C).
Cook the macaroni according to package instructions until al dente, then drain and set aside.
In a large saucepan, melt the butter over medium heat.
Whisk in the flour and cook for about 1-2 minutes before adding milk.
Gradually add the milk, whisking constantly until the mixture becomes thickened and bubbly, about 5-7 minutes.
Stir in Dijon mustard, garlic powder, salt, and pepper, mixing until well combined.
Remove from heat and mix in the shredded cheddar, Gruyère, mozzarella, and half the Parmesan cheese until melted and smooth.
Pour the cheese sauce over the cooked macaroni and stir to combine evenly.
Transfer the mac and cheese mixture to a greased baking dish, smoothing it out evenly.
In a small bowl, mix the panko breadcrumbs with melted butter and the remaining Parmesan cheese; sprinkle over the top of the mac and cheese.
Bake for 25-30 minutes, or until the top is golden brown and the sauce is bubbly.
Remove from the oven and let it cool for a few minutes before serving; garnish with chopped parsley if desired.
