Preheat the oven to 350 degrees F. Line a baking sheet with parchment paper and place a cooling rack on top. Mist the rack lightly with cooking spray.
Combine the beef, egg, 1/4 cup ketchup, the onion, breadcrumbs, Worcestershire sauce, parsley, thyme, 1/2 teaspoon each salt and pepper, and the garlic powder in a large bowl. Use your hands to mix everything together.
Shape tablespoonfuls of the meat into small, thin patties, about 2 inches in diameter. Place on the prepared rack on the baking sheet. Brush the patties with the remaining 1/4 cup ketchup and bake about 20 minutes.
Meanwhile, spread mayonnaise on one side of each bun. Serve each meatloaf slider on a prepared bun.