Preheat Oven: Preheat your oven to 350°F (175°C).
Cook Onions: In a cast iron skillet, cook the diced onions in butter over medium-low heat until they are tender. Let them cool completely.
Prepare Meatloaf Mixture: In a medium bowl, combine the eggs, milk, breadcrumbs, ground beef, cooked onions, chili sauce, Italian seasoning, parsley, salt, and pepper to the bowl. Mix until just combined, do not overmix!
Shape and Bake: Form the ground beef mixture back into the cast iron skillet. Bake for 40 minutes.
Prepare Sauce: While the meatloaf is baking, combine the chili sauce, ketchup, mustard, and brown sugar in a small bowl.
Add Sauce and Finish Baking: After 40 minutes of baking, spread the sauce mixture over the meatloaf. Return the meatloaf to the oven and bake for an additional 10-15 minutes, or until the internal temperature reaches 160°F (71°C). For a caramelized top, broil for 1-2 minutes if desired.
Rest and Serve: Let the meatloaf rest for 10 minutes before serving.