Bring a large pot of salted water to a boil and cook 12 ounces pasta shells according to package directions until al dente. Drain and set aside.
Heat a large skillet over medium-high heat. Add 1 pound ground beef and cook for 5 to 7 minutes, breaking it up as it cooks.
Season the beef with 1 teaspoon salt, ½ teaspoon black pepper, 1 teaspoon smoked paprika, and 1 teaspoon garlic powder. Once browned, remove the beef from the skillet and set aside.
Reduce the heat to medium and add 3 tablespoons butter to the same skillet. Add 1 diced onion and cook for 3 minutes, until softened. Stir in 4 minced garlic cloves and cook for 30 seconds, just until fragrant.
Sprinkle 2 tablespoons flour over the onion and garlic mixture. Stir constantly for 1 minute.
Slowly pour in 1½ cups beef broth, whisking as you go, and let the sauce simmer for 3 to 4 minutes, until slightly thickened. Stir in 1 cup heavy cream, ½ cup milk, 1 tablespoon Worcestershire sauce, and 1 teaspoon Italian seasoning. Reduce the heat to low and simmer for 3 minutes.
Add 1 cup shredded cheddar cheese and ½ cup grated Parmesan cheese, stirring until fully melted and smooth.
Return the cooked beef to the skillet along with the drained pasta. Toss everything together until well coated in the sauce.
Let it cook for 2 to 3 minutes, until hot and extra creamy. Remove from heat and garnish with chopped parsley if desired.