Preheat the oven to 350°F. Line a rimmed baking sheet or baking pan with aluminum foil and lightly spray with cooking spray.

In a small skillet, cook the diced onion with butter over medium-low heat until soft and tender. Set aside to cool completely.

In a large mixing bowl, whisk together the eggs, milk, and breadcrumbs. Let the mixture rest for 5 to 10 minutes so the breadcrumbs absorb the liquid.

Add the ground beef, cooled onions, ketchup or chili sauce, Italian seasoning, parsley, salt, and black pepper.

Gently mix the ingredients until just combined. Avoid overmixing to keep the Best Meatloaf Recipe tender and juicy.

Shape the meat mixture into an 8x4-inch loaf and place it on the prepared baking pan.

Bake uncovered for 40 minutes.

While the meatloaf bakes, stir together the chili sauce, ketchup, and brown sugar in a small bowl.

Spread the sauce evenly over the meatloaf and return it to the oven.

Bake for an additional 15 to 30 minutes or until the internal temperature reaches 160°F.

Broil for 1 to 2 minutes if you want a caramelized topping.

Allow the meatloaf to rest for 10 minutes before slicing and serving.