Preheat oven to 200 degrees C. Grease a 23x33cm baking dish with 2 tablespoons butter. Fry bacon until evenly brown. Remove, dice and set aside.
Place frozen spinach in a medium saucepan with 4 tablespoons water. Bring water to a boil, then reduce heat to medium and cook spinach covered 10 minutes. Uncover and stir. Remove from heat and drain well. Remove stems from mushrooms. Arrange caps in the baking dish. Finely dice stems.
Melt 60g butter in a medium saucepan over medium heat then mix in onion and garlic. Cook until tender, then mix in bacon, spinach, diced mushroom stems and thickened cream. Bring to a simmer. Remove from heat and mix in Parmesan cheese.
Stuff mushroom caps generously with the mixture. Bake in preheated oven 30 minutes until lightly browned.