Preheat the oven to 350°F.

Lightly grease a casserole dish with butter or nonstick cooking spray.

In a large skillet over medium heat, cook the ground beef and chopped onions until the beef is fully browned and the onions are softened.

Drain any excess grease from the skillet and set the beef mixture aside.

Peel the potatoes and slice them into 1/4 to 1/2-inch slices.

Arrange a layer of sliced potatoes in the bottom of the prepared casserole dish.

Spread half of the cooked ground beef and onion mixture over the potato layer.

Add another layer of sliced potatoes over the beef mixture.

Spread the remaining ground beef and onion mixture over the potatoes.

Continue layering any remaining potatoes evenly over the top.

To prepare the cheese sauce, melt the salted butter in a saucepan over medium heat.

Add the all-purpose flour, salt, and black pepper.

Whisk until the flour is fully incorporated into the melted butter.

Gradually add the whole milk while whisking continuously.

Continue cooking and stirring until the sauce thickens.

Remove the saucepan from the heat.

Add the shredded cheddar cheese and stir until the cheese is fully melted and the sauce is smooth.

Pour the cheese sauce evenly over the layered potatoes and ground beef.

Use a spoon or spatula to spread the sauce so the potatoes are well coated.

Cover the casserole dish tightly with aluminum foil.

Bake for approximately 90 minutes, or until the potatoes are tender and fully cooked.

Remove the casserole from the oven and allow it to rest for several minutes before serving.

Serve warm.