Step 1: Preheat your oven to 375°F (190°C). Lightly grease a 9×13 inch baking dish and set aside. Prepare all your ingredients by dicing the onion, mincing the garlic, and thinly slicing the potatoes.
Step 2: In a large skillet or Dutch oven, heat the olive oil over medium-high heat. Add the ground beef and cook, breaking it apart with a spoon, until no pink remains. Drain any excess fat.
Step 3: Add the diced onion to the skillet with the beef and cook until softened, about 5-7 minutes. Stir in the minced garlic and cook for another minute until fragrant.
Step 4: Stir in the cream of mushroom soup, milk, salt, black pepper, paprika, and garlic powder into the beef mixture. Mix well until a creamy sauce forms. Bring to a gentle simmer, then remove from heat.
Step 5: Spread about 1/3 of the beef and sauce mixture evenly on the bottom of the prepared baking dish.
Step 6: Arrange half of the thinly sliced potatoes in an even layer over the beef mixture.
Step 7: Spread half of the remaining beef mixture over the potatoes. Sprinkle with 1.5 cups of the shredded cheddar and 0.5 cup of the Monterey Jack cheese (if using).
Step 8: Layer the remaining half of the potatoes over the cheese.
Step 9: Top with the rest of the beef mixture, spreading it out evenly. Finish with the remaining 1.5 cups of cheddar cheese and 0.5 cup of Monterey Jack cheese.
Step 10: Cover the baking dish tightly with aluminum foil. Bake for 45-60 minutes, or until the potatoes are fork-tender.
Step 11: Remove the foil and continue to bake for another 15-20 minutes, or until the cheese is melted, bubbly, and golden brown on top.
Step 12: Let the casserole rest for 10-15 minutes before serving. Garnish with fresh parsley, if desired, and enjoy your delicious cheesy ground beef potato casserole!