In a large bowl, combine the ground beef, bread crumbs or oats, parsley, pepper, salt, and eggs. Using your hands (you can wear latex gloves if you’re squeamish) squish everything together until evenly incorporated.
Lay the meat mixture out onto a piece of wax paper and press it out into a roughly 10×12-inch rectangle.
Set aside ½ cup of the cheese and ½ cup of the onions. Sprinkle the remaining 1½ cups cheese and onions evenly over the ground beef. Roll up like a jelly roll, starting from the short end, lifting the wax paper or plastic wrap as you roll. Seal the end completely, and place the loaf seam-side down into a 9×13-inch baking pan.
Place the meatloaf in the oven and bake at 350 degrees for 45 minutes. While the meatloaf is baking, prepare the sauce. For the sauce: Whisk together the sauce ingredients in a small pot, bring the mixture to a boil, and simmer until thickened. Remove from heat and set aside
Remove the pan from the oven, pour the prepared sauce over the meatloaf, and sprinkle the top with the reserved onions and cheese.
Return the meatloaf to the oven for another 15-20 minutes, or until the meat is no longer pink on the inside. Place under the broiler for 2-3 minutes, or until the cheese on top is browned and bubbly.Remove from oven and allow to stand 10 minutes. Slice and serve. Garnish with fresh parsley, if desired.