Preheat the oven to 350 degrees.
Cook the bacon in a skillet over medium heat until cooked through and crispy.
Remove from the skillet and place on a paper towel to drain grease. Chop into crumbles.
Remove all but 1 teaspoon of bacon grease from the skillet.
Add the shallot and cook, stirring often, for 1 minute.
Add the spinach and cook, stirring often, for 2 minutes.
Add the minced garlic and cook, stirring constantly, for 1 minute.
Remove from the heat and pour into a bowl.
Once the spinach mixture cools a bit, add the cream cheese, Gruyere, panko crumbs, and most of the bacon bits; stir until well combined.
Taste and season with sea salt and freshly cracked pepper, to taste, if needed.
Carefully remove the stems from each mushroom.
Place the mushrooms in a cast iron skillet (or baking dish) and drizzle with some olive oil; toss to coat evenly.
Place spoonfuls of the bacon & spinach mixture into each mushroom, making sure to gently press the mixture down into the mushrooms.
Sprinkle the top of each mushroom with a bit more shredded Gruyere.
Place into the oven and bake for 15-18 minutes.
Remove from the oven and sprinkle with the remaining bacon bits.
Serve immediately. Enjoy.