Preheat the oven to 400 degrees F (200 degrees C). Very generously butter a 9x13-inch baking dish.

Cook bacon in a large, deep skillet over medium-high heat until evenly brown, about 10 minutes. Drain, crumble, and set aside.

Place frozen spinach in a medium saucepan. Add 1/4 cup water and bring to a boil. Reduce the heat to medium, cover, and cook for 10 minutes. Uncover and stir, then remove from the heat and drain.

Remove stems from mushrooms. Arrange caps in the prepared baking dish. Finely chop stems.

Melt 3 tablespoons butter in a medium saucepan over medium heat. Add onion and garlic; cook and stir until onion is translucent, about 5 minutes. Stir in spinach, bacon, and mushroom stems, then pour in 1/3 cup plus 1 tablespoon cream; bring to a boil. Remove from the heat and stir in Parmesan, salt, and pepper.

Stuff mushroom caps generously with spinach mixture. Drizzle 2 tablespoons melted butter over the top.