Fry the bacon until crispy. Drain and reserve two tablespoons of the bacon fat. Finely chop the bacon and set aside. Roughly chop the baby spinach and set aside.
Add one tablespoon of the bacon fat to a skillet and place over medium high heat. Add the diced onion and sauté for 3 to 4 minutes until the onion is softened and beginning to brown. Add the garlic and sauté briefly. Add the spinach to the skillet and cook just until it wilts. Transfer the spinach mixture to a bowl and let cool.
In a mixing bowl, combine the softened cream cheese, chopped bacon, shredded parmesan cheese, 1/2 tsp kosher salt and 1/4 tsp pepper. Blend together. Add the cooled spinach mixture blend together.
Preheat the oven to 375° F. Prepare the mushrooms by first removing the stems. Use a teaspoon to scoop out the gills from inside the mushroom caps. Arrange the mushrooms on a baking sheet. Generously stuff the mushrooms with the filling.
In a small bowl, combine the Panko crumbs, parsley, grated parmesan, 1/4 tsp kosher salt and a pinch of pepper. Add the remaining tablespoon of bacon fat and mix together.
Spoon the crumbs over each mushroom. Bake for 25 to 35 minutes until the mushrooms are fully cooked and the filling is hot.