Preheat your oven to 350°F (175°C). Peel and thinly slice the russet potatoes. Chop the onion.
In a skillet, heat 1 tablespoon of olive oil over medium heat. Add the ground beef and chopped onion, cooking until the beef is browned and the onion is translucent. Drain any excess grease. Season the beef and onion mixture with garlic powder, paprika, salt, and pepper.
In a separate bowl, mix together the condensed cream of mushroom soup and milk until smooth. In a greased 9x13 inch baking dish, arrange a layer of sliced potatoes. Top the potatoes with half of the beef and onion mixture, then pour half of the soup mixture over it. Sprinkle with 1 cup of shredded cheddar cheese. Repeat the layers with the remaining potatoes, beef mixture, soup mixture, and cheese.
Cover the baking dish with aluminum foil and bake in the preheated oven for 1 hour. Uncover and bake for an additional 30 minutes, or until the potatoes are tender and the top is golden and bubbly.
Remove the casserole from the oven and let it sit for a few minutes. Garnish with chopped fresh parsley before serving.