Preheat the oven to 350°F. In a large bowl, whisk together the eggs, milk, Worcestershire sauce, salt, garlic powder, onion powder, and black pepper. Add the oats and stir them in so they start to soften. If time allows, let the mixture sit for a few minutes before adding the beef. It helps the oats absorb moisture and keeps the texture from getting crumbly. Stir in the grated onion. Add the beef to the bowl and use your hands or a fork to gently mix until just combined.
Transfer the beef mixture to a greased loaf pan.
Bake uncovered for about 60 minutes, or until the meatloaf reaches an internal temperature of 160°F and the juices run clear. Tilt the pan to drain off the grease; spread ketchup on top, and return the meatloaf to the oven for 10 more minutes.
Let stand for at least 5 minutes before slicing.
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Notes
Use quick oats for the softest texture; avoid steel-cut oats.
Add herbs to the meat mixture, such as basil, oregano, rosemary, chives, parsley, or thyme.
Add more vegetables, but sauté them in a skillet first.
85% lean ground beef works well. Anything leaner can dry out, while fattier blends can make the loaf greasy.
Don't overmix the meat or it will get dense or tough.
Bake in a loaf pan or shape it free-form on a sheet pan.
Keep leftovers in an airtight container in the fridge for up to 4 days, or freeze for up to 3 months.
Reheat slices in a 325°F oven for about 10 minutes, or microwave individual portions until hot.