Preheat oven to 350℉. Grease or spray an 8" x 8" baking dish.

Heat oil in a large skillet on MEDIUM heat. Add beef and onion; cook until meat has cooked through and browned. Drain/discard grease. Add garlic powder, paprika, salt, pepper and water to meat. Cook (and stir) 1-2 minutes, until water has cooked out and spices coat the beef. Taste; add additional salt (if necessary).

PREP: In a medium bowl, stir mushroom soup and milk until well-combined. Set bowl aside. Wash potatoes well, leaving the peel ON. Slice potatoes very thin (1/8"-1/4") with sharp knife (or mandolin). Set slices aside. Grate cheddar cheese; set it aside, also. Now it's time to make the casserole!

Place HALF the sliced potatoes in a single layer on the bottom of greased baking dish, covering as much of the bottom as possible. Evenly distribute HALF the beef mixture over potatoes. Spread HALF the mushroom soup mixture over ground beef. Top with HALF the grated cheese. Repeat process one more time, layering potatoes, meat, mushroom soup, and remaining cheese. 

Cover casserole with aluminum foil; place on middle rack in preheated oven. Bake (covered) at 350℉ for 45 minutes, then remove foil. Bake (uncovered) an additional 20 minutes until casserole is lightly browned and bubbly around the edges. Remove casserole from oven; let it rest 4-5 minutes to "firm up" slightly before slicing. Garnish with parsley, if desired. Cut into 4 portions, and serve!