Preheat oven to 350°F (175°C).

Cook lasagne noodles according to package directions.

Drain the cooked noodles.

Cut the onion into small pieces.

Butter a 13 x 9-inch baking pan.

Layer noodles on the bottom of the prepared pan.

Spread cooked mashed potatoes over the noodles.

Sprinkle the diced onion over the potatoes.

Top with a layer of shredded Cheddar cheese.

Repeat layers of noodles, potatoes, onion, and cheese until all ingredients are used.

Bake in the preheated oven for 30 to 35 minutes, or until golden brown and bubbly.

Let cool for 5 minutes before serving.