Preheat oven to 375 degrees F (190 degrees C).

Over medium-low heat melt butter in a large saucepan. Add onions and cook slowly, stirring occasionally, until onions are soft.

In a small bowl, beat together eggs and cream. Stir in cheese. Spread onions in bottom of pastry shell. Pour egg mixture over onions.

Bake in preheated oven for 30 minutes, or until eggs have set.