In a large saucepan or Dutch oven over medium-high heat, add beef and onion and stir to combine. Cook until the meat is mostly browned, about 5 minutes. Drain well.
To the pot with the drained beef, add pasta, chicken broth, tomato sauce, ketchup, brown sugar, white vinegar, Worcestershire sauce, and dried mustard. Bring to a boil, reduce heat, and simmer uncovered until pasta is tender, about 10 to 12 minutes.
Season to taste with salt and pepper (I like 1/2 teaspoon salt and 1/4 teaspoon pepper). Serve with cheddar cheese and hot sauce if desired and garnish with fresh parsley.
Notes
Ground beef: Or substitute ground turkey or venison.
Onion: Finely chopped celery and a green bell pepper are great too!
Brown sugar: Even a tablespoon is too much for some people, so just leave it out if it sounds horrible to you.
Worcestershire sauce: In a pinch, you can substitute soy sauce.
Ground mustard: Or substitute an equal amount of prepared yellow mustard or Dijon mustard.
Yield: This Sloppy Joe Casserole makes over 8 cups of casserole (not including the cheese), enough for 8 servings, 1 cup each.
Storage: Store leftovers covered in the refrigerator for up to 4 days.