Tuna Italiano

1/2 cup chopped onion
2 tablespoons butter
1/4 teaspoon oregano, crushed
1 can cream of mushroom soup
2 cups cooked elbow macaroni
1/2 cup chopped canned tomatoes
1 large can tuna, drained and flaked

Saute onion in butter with oregano until tender. Blend in soup; add remaining ingredients. Heat, stirring occasionally.

Serve with grated Parmesan cheese.