Preheat oven to 350°F. Cut the peppers in half lengthwise. Clean out the seeds and wash the peppers. Pat dry with a paper towel.
Combine the ground beef, grated onion, eggs, bread crumbs, 1 tablespoon of ketchup, salt and pepper. Fill the pepper halves with the mixture.
Combine the ½ cup ketchup and balsamic vinegar. Brush each pepper half with the mixture. Baste the peppers again after 30 minutes of baking.
Put the olive oil in a baking dish or pan and then place the peppers in the pan. Bake for about 45 minutes to one hour.