Preheat oven to 350°F.
In a large bowl, stir together eggs, oats or breadcrumbs, ketchup, parsley, Worcestershire sauce, salt, thyme, garlic powder, and black pepper. Set aside.
In a microwave-safe dish, microwave green bell pepper and onion covered at HIGH power for about 1 ½ - 2 minutes, or until softened. Set aside to cool slightly for a few minutes.
Grease a standard 12-cup muffin tin with nonstick cooking spray. Set aside.
Add the onion mixture to the large bowl with the egg mixture. Stir to combine. Using your hands or a fork, gently mix in the ground beef, just until all of the ingredients are combined. Do not overmix.
Divide evenly between the 12 prepared muffin cups, pressing down gently so that the meat mixture fills each cup. Bake for 15 minutes.
Meanwhile, prepare the meatloaf sauce by whisking together the ketchup, brown sugar, and vinegar in a small bowl.
Remove the meatloaf from the oven. Use a paper towel to blot the meatloaf, soaking up any visible excess grease. Spoon the sauce over each mini meatloaf. Return to oven for an additional 10-15 minutes, or until the meatloaves are cooked through and a thermometer registers 160°F.