Preheat oven to 375 degrees F (190 degrees C). Spray a 9 inch pie pan with non-stick cooking spray.
Melt 2 tablespoons butter in a large saucepan over medium-high heat. Add onions and cook, stirring occasionally, until onions are soft. Squeeze extra water out of the spinach and add the spinach to the skillet. Continue cooking until the spinach is heated through and the extra water evaporates. Spoon mixture into pie pan.
In a large bowl, beat together eggs and cream. Stir in salt, pepper and nutmeg. Pour into pie pan, top with shredded Swiss cheese. dot with remaining 1 tablespoon of butter.
Bake 25 to 30 minutes until set and golden brown. Cool 5 minutes before serving.