1. Heat your oven to 428 degrees Fahrenheit (220 degrees Celsius).
2. In the bowl of a food processor, combine butter, garlic, shallots, parsley, and lemon juice. Process the mixture until smooth, scraping the sides once to prevent any big unprocessed chunks.
3. Crumble the ground beef into a large bowl using your fingers and season it with salt and pepper. Scrape the butter mixture onto the beef along with breadcrumbs, eggs, and gently mix everything together with your hands until evenly combined. Transfer the mixture to aluminum foil and shape it into an 8 1/2-by-4 1/2-inch rectangle, 1 1/2 inches thick. If you're using the vinegar, stir it into the ketchup inside a small bowl.
4. Heat an ovenproof skillet (at least 10 inches in diameter) over medium-high heat for 2 minutes. Pour the oil into the skillet and swirl it around to evenly coat it. Use the foil as a carrier, gently flip the meatloaf into the hot skillet, and remove the foil. Spread the ketchup evenly over the meatloaf (avoid letting any fall down the sides), and then put the skillet inside the oven. Bake it for 28 to 30 minutes, or until the bottom is crusty and an instant-read thermometer inserted in the center of the meatloaf reads 158 degrees Fahrenheit (70 degrees Celsius).
5. Remove the skillet from the oven and let the meatloaf cool in the pan for 5 minutes. Serve it from the skillet or use two metal spatulas to transfer it to a serving platter, then cut it into 1/2-inch thick slices, and serve it with some green peas and some lemon wedges on the side. Garnish it with some additional parsley if you'd like.