Heat the olive oil in a large skillet over medium heat. Add the sliced onions and cook, stirring often, until they turn golden and soft, about 15 minutes. Set aside to cool slightly.
In a large mixing bowl, combine the ground beef, chopped celery, breadcrumbs, egg, milk, salt, black pepper, garlic powder, Worcestershire sauce, and the cooled caramelized onions. Mix with your hands until everything is evenly blended.
Shape the mixture into a loaf and place it on a baking sheet lined with parchment paper. Spread the ketchup evenly over the top of the loaf.
Bake at 350 degrees F for 50 to 55 minutes, or until the internal temperature reaches 160 degrees F. Let the meatloaf rest for 5 minutes before slicing.