In an oven-safe enameled cast iron pot, braiser pan, or deep 10-inch skillet over medium-high heat, cook 1 1/2 pounds ground beef, seasoning with 1/2 teaspoon salt, 1/2 teaspoon black pepper, and 1/2 teaspoon garlic powder. Cook until nearly browned, about 4 to 5 minutes. Add the diced 1 medium white onion and cook until soft and translucent, about 5 minutes.
Add 2 tablespoons salted butter and let it fully melt. Stir in 1/4 cup all-purpose flour and cook for 1 to 2 minutes, stirring frequently.
Add 8 ounces rotini pasta, 4 cups beef broth, 1/2 cup heavy cream, 1/4 cup ketchup, 2 tablespoons Worcestershire sauce, and 2 tablespoons yellow mustard. Stir well and bring to a simmer. Cover and cook for 10-12 minutes, stirring occasionally, until the pasta is al dente and the sauce has thickened.
Remove from heat and top evenly with 4 hamburger buns. Sprinkle with 2 cups shredded cheddar cheese and 1/2 cup sliced dill pickles (if using).
Transfer to the preheated oven and bake, uncovered, for 15 to 20 minutes, or until the cheese is melted and the bread cubes are golden brown.
Remove from oven, let sit for about 5 minutes and enjoy!