01 - Preheat oven to 425°F. Toss broccoli florets with olive oil, sea salt, and black pepper on a baking sheet. Roast for 15 to 18 minutes until crisp and lightly browned. Remove from oven and set aside. Lower oven temperature to 375°F.
02 - Boil a large pot of salted water. Cook elbow macaroni until al dente, approximately 7 to 8 minutes. Drain and set aside.
03 - In a medium saucepan over medium heat, melt butter. Whisk in all-purpose flour and cook for 1 minute. Gradually pour in whole milk while whisking continuously. Cook until sauce thickens, about 4 to 5 minutes.
04 - Remove saucepan from heat. Stir in grated cheddar, Gruyère, Dijon mustard, garlic powder, black pepper, and kosher salt until fully melted and smooth.
05 - In a large bowl, fold together cooked macaroni, roasted broccoli, and cheese sauce until uniformly coated.
06 - Transfer mixture into a greased 9x13-inch baking dish.
07 - Combine panko breadcrumbs, grated Parmesan cheese, and melted butter in a small bowl. Evenly sprinkle over macaroni mixture.
08 - Bake in preheated 375°F oven for 20 to 25 minutes until topping is golden and sauce is bubbling.
09 - Allow to rest for 5 minutes before serving to set flavors and texture.