Preheat the oven to 400 degrees F. Line a baking sheet with tin foil then top with a wire rack and set aside.
For the potatoes: Fill a medium sauce-pot with cold water and salt liberally. Add the peeled, diced potatoes into the cold water and cover with a tight-fitting lid until the water begins to boil. Remove the lid and continue to boil the potatoes another 12 to 15 minutes or until fork tender and ready to mash.
While the potatoes are boiling, start on the meatloaf.
For the Meatloaf: Whisk together the milk and egg in a small bowl; add the oatmeal and stir to combine. Set aside.
Mince the onion, garlic, carrot, celery, and parsley together in a food processor. Combine the oatmeal mixture, vegetable mixture, ground beef and pork together with the ketchup, mustard, Worcestershire, salt and ground pepper. Mix gently with just your fingertips together in a large bowl. This keeps the mixture soft and moist instead of compacted and dense. Form the mixture into two loaves; place on the prepared baking rack and bake for 25 minutes. When done, the internal temperature of the meatloaf should be 160 degrees F and the juices should run clear.
When the potatoes are fork tender, drain them well and return the potatoes to the pan over low heat. This evaporates any excess water. Add 6 tbsp butter (3/4 stick) and milk. Mash the potatoes until smooth - if you prefer, use a ricer for silky smooth potatoes. Add the cheese and stir to combine. Season with salt and pepper to taste. Add scallions (optional) and mix to combine.
Spoon the mashed potatoes over the top of the two meatloaves. Dot with small pads of the remaining 2 tbsp butter. Turn the oven up to broil. Broil the meatloaf for an additional 3-5 minutes until the potatoes have browned slightly. Remove from the oven and allow the loaves to rest for 10 minutes prior to slicing and serving.
Optional garnishes: garnish with chopped parsley, extra scallions, sliced chives and/or serve with brown gravy.
Notes
This recipe is easily doubled. Try any leftovers as a sandwich on fresh bread. :)