Cut the zucchini lengthwise and remove the seeds.

Grease an ovenproof dish and arrange the zucchini in it.

Chop the onions.

Saute the chopped onions in olive oil in a large skillet.

Set the sauteed onions aside.

Preheat oven to 350°F (175°C).

Chop the tomatoes.

In a large saucepan, simmer the corn and chopped tomatoes for 10 minutes.

Let the corn and tomato mixture cool.

In a bowl, stir together the cooled corn and tomato mixture, egg, breadcrumbs, marjoram, and the reserved sauteed onions.

Spoon the filling into the zucchini halves.

Bake in the preheated oven for 40 minutes.