Preheat oven to 375F and line a loaf pan (9“ x 5”) with parchment paper; spray with cooking spray
Finely dice the onion. Heat oil in a skillet over medium-high and add onion. Add some salt (maybe 1/2 tsp kosher salt) and stir frequently, cooking until onion is well-browned and soft, about 30 minutes
While the onion is cooking, combine the ground beef, eggs, garlic, 3 tbsp ketchup, parsley, panko, 1/2 tbsp salt (or to taste), Italian seasoning, 1/2 tsp black pepper, and paprika. Mix well
Add onions to the bowl of ground beef and mix well to combine
Add beef mixture to loaf pan, pressing into the pan well. Put in oven for 55 minutes
While the meatloaf is cooking, combine the sauce ingredients: 3/4 cup ketchup, white vinegar, brown sugar, garlic powder, onion powder, and salt and pepper to taste
After the 55 minutes, remove meatloaf from oven and put sauce on top. Put back in oven and continue baking for another 25 minutes
Rest meatloaf 10 minutes before slicing