Preheat the oven to 375 degrees Fahrenheit.
Cook the pasta: Bring a large pot of salted water to a boil. Add the pasta and cook according to package instructions until al dente. Drain the pasta and set aside.
Prepare the Sloppy Joe Sauce: In a medium bowl, whisk together the ketchup, brown sugar, Worcestershire sauce, apple cider vinegar, yellow mustard, garlic powder, onion powder, salt, black pepper, and dried oregano. Mix well and set aside.
Cook the beef and onions: In a large oven-safe skillet, cook the ground beef and onion over medium heat until the meat is browned and the onion is soft, about 8 minutes. Drain off the excess grease.
Add the garlic: Stir in 3 cloves of minced garlic and cook for 30 seconds, until fragrant.
Simmer the sauce: Pour the prepared Sloppy Joe sauce into the skillet with the cooked beef and onions. Stir well and let the mixture simmer on low for about 5 minutes to allow the flavors to blend. Taste the sauce and adjust the seasoning with additional salt and pepper, if needed.
Add the cooked pasta: Add the cooked pasta shells to the skillet and stir to coat the pasta in the sauce.
Assemble: Transfer the pasta and meat mixture to a greased 9x13-inch baking dish (transferring to a different dish is unnecessary if you're already using an oven-safe skillet). Sprinkle with 2 cups of shredded cheddar cheese evenly over the top.
Bake and serve: Place the casserole in the oven and bake for 15-20 minutes or until the cheese is melted and bubbly. Garnish with fresh parsley, if desired.