Preheat the oven to 350°F (175°C).
Cook the elbow macaroni according to package instructions until al dente. Drain and set aside.
In a large skillet over medium heat, add the ground beef and diced onion. Cook until the beef is browned and the onion is softened, about 6-8 minutes. Drain excess fat.
Add the minced garlic, salt, black pepper, and smoked paprika to the beef mixture. Stir and cook for 1 more minute. Remove from heat and set aside.
In a medium saucepan, melt the unsalted butter over medium heat.
Whisk in the all-purpose flour and cook for 1-2 minutes, stirring constantly, to make a roux.
Gradually whisk in the whole milk, continuing to whisk until the mixture is smooth and begins to thicken, about 5-7 minutes.
Remove the sauce from heat and stir in the shredded sharp cheddar cheese, shredded mozzarella cheese, grated Parmesan cheese, dry mustard powder, and cayenne pepper until the cheese is fully melted and the sauce is smooth.
Combine the cooked macaroni, ground beef mixture, and cheese sauce in a large mixing bowl. Stir until well combined.
Transfer the mixture to a greased 9×13-inch baking dish and spread evenly.
Sprinkle the plain breadcrumbs evenly over the top of the casserole.
Bake in the preheated oven for 20-25 minutes, or until the top is golden brown and the casserole is bubbly.
Remove from the oven and let cool for 5 minutes before serving.