Preheat oven to 350°F. Line a rimmed baking pan with foil and spray with cooking spray.
Sauté onion in butter over medium-low heat until soft. Cool completely.
In a bowl, mix eggs, milk, and breadcrumbs. Let sit 5–10 minutes.
Add ground beef, onions, ketchup or chili sauce, seasoning, parsley, salt, and pepper.
Mix gently to combine.
Shape into an 8"x4" loaf on the pan. Bake for 40 minutes.
Mix glaze ingredients. Spread over meatloaf.
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