Take the hamburger, diced onion, oatmeal, eggs, and onion soup mix and mix well by hand. Add a dash of milk if needed to help blend. Separate in half.
Place half of the meatloaf mixture in the bottom of a roaster and shape into an oval. Sprinkle the cheese on this half and cover with the additional meatloaf mixture. Press edges to seal. Drizzle the top of the meatloaf and cover.
Bake covered at 350 degrees for 1 hour. Take the cover off and bake an additional 30 minutes. Let sit for a few minutes before cutting to allow the cheese to set slightly.
As an added bonus, you can also surround the meatloaf with baby carrots and halved baby red potatoes for an easy and delicious side. They cook right along with the meatloaf and become glazed by the bbq sauce...yum!