Preheat the oven to 350 degrees F (175 degrees C). Grease a 7x11-inch baking dish.

Heat a large skillet over medium-high heat. Add beef; cook and stir until browned and crumbly, 5 to 7 minutes. Drain and discard grease.

Combine 1 can mushroom soup, mushrooms, 3/4 cup milk, seasoned salt, and black pepper in a bowl; fold in potatoes.

Spread 1/2 potato mixture into the prepared baking dish; top with Cheddar cheese. Spoon 1/2 ground beef over cheese layer; top with remaining 1/2 potato mixture. Add remaining 1/2 ground beef on top.

Combine remaining 1 can mushroom soup and remaining 1/4 cup milk; pour over ground beef layer.

Bake in the preheated oven until casserole is bubbling, about 1 hour 20 minutes. Sprinkle French-fried onions over casserole; bake 5 minutes more.