Heat the oven to 350°F and line a rimmed baking sheet with parchment paper or foil. The lining makes cleanup easier and keeps the shaped loaf from sticking to the pan.

Warm the olive oil in a medium skillet over medium heat, then cook the chopped onion for 5 to 7 minutes, stirring now and then. Move the onion to a plate when it is soft and golden, then let its fragrant steam fade as it cools.

Place the ground beef, cooled onion, eggs, garlic, ketchup, parsley, panko breadcrumbs, milk, salt, Italian seasoning, and black pepper in a large bowl, then mix gently until combined. Stop when the mixture looks even and feels soft, because continued mixing makes the baked loaf dense.

Transfer the mixture to the prepared sheet and shape it into an 8-inch-long, 4-inch-wide, 3-inch-tall loaf. Keep the shape even from end to end so the center and edges finish cooking at a similar pace.

Bake the loaf uncovered for 40 minutes at 350°F. The outside should begin to brown while the top remains ready to hold the glaze.

Stir together the ketchup, white vinegar, brown sugar, garlic powder, and onion powder in a small bowl. The sauce should look smooth and glossy, with the brown sugar fully dispersed.

Spread the sauce over the partially baked loaf and return it to the oven for 20 minutes, until an instant-read thermometer in the center reads 160°F. The glaze should bubble at the edges and deepen in color without scorching.

Rest the meatloaf for 10-15 minutes before cutting. The loaf firms as it rests, making each slice cleaner and less likely to crumble.