Brown the Ground Beef and Onions: In a large skillet, heat oil over medium-high heat. Add ground beef and cook until browned, about 5-7 minutes. Stir in diced onion and cook until translucent, about 3-4 minutes.

Combine the Mixture: Drain excess fat from the skillet. Reduce heat to low and add cream of mushroom soup, milk, garlic powder, salt, and pepper. Stir until combined.

Prepare the Crockpot: Grease the slow cooker or use a liner. Layer half of the sliced russet potatoes on the bottom.

Layer the Beef Mixture and Cheese: Pour half of the beef mixture over the potatoes, then sprinkle with half of the cheddar and Monterey Jack cheese.

Repeat the Layering: Layer the remaining sliced potatoes, then the rest of the beef mixture and top with the remaining cheese.

Cook: Cover and cook on LOW for 6-8 hours or HIGH for 3-4 hours until potatoes are fork-tender.