1 - Preheat oven to 400 degrees F (200 degrees C).

2 - Grease a 10x14x2-inch baking pan.

3 - Heat margarine in a skillet over medium heat; stir in onion.

4 - Cook and stir until onion has softened and turned translucent, about 5 minutes.

5 - Reduce heat to low and continue to cook and stir until onion is very tender and dark brown, about 40 minutes more.