In a large bowl, combine beaten egg, breadcrumbs, salt, pepper, and ground beef. Mix lightly but thoroughly.
Lay out a large sheet of aluminum foil and pat mixture out onto foil in a 8x14-inch rectangle.
In a separate bowl, mix together filling ingredients - mashed potatoes, hardboiled eggs, Miracle whip, Parmesan cheese, celery, green onion, salt, ground mustard, and black pepper. Spread over meatloaf mixture to within 1-inch of edges.
Roll up meatloaf jelly roll style, starting with short side, but do not roll up foil. Press seam and ends to seal and place on a plate. Cover and chill overnight.
Preheat oven to 350°F.
Cut roll into 6 slices and place slices on a wire rack on a pan, cut side up. Bake until a thermometer reads 160°F, about 30-35 minutes.
In a small bowl, mix together sauce ingredients - Miracle Whip, milk, and green onion and serve over meatloaf.