Preheat oven to 350°F (175°C).
Prepare the meatball mixture by combining ground meat, egg, panko bread crumbs, Parmesan cheese, grated onion, Montreal steak seasoning, trinity seasoning, salt, and pepper in a large bowl. Mix until well combined.
In a casserole dish, layer the halved or quartered potatoes and drizzle with olive oil. Toss to coat the potatoes evenly.
Roll the meat mixture into golf ball-sized meatballs and place them over the potatoes in the casserole dish.
Sprinkle the sliced onions over the meatballs and potatoes.
In a small bowl, whisk together beef gravy, ketchup, Worcestershire sauce, Cajun seasoning, and Lipton onion soup mix until smooth.
Pour the sauce mixture evenly over the meatballs and potatoes.
Bake the casserole covered at 350°F for 1 hour, or until the potatoes are tender and the meatballs are fully cooked.
Serve warm and enjoy the comforting flavors of Salisbury steak meatballs and potatoes.