Preheat oven to 400˚F. Line a sheet-pan with parchment paper.
Cut the ends from the stems of the mushrooms and discard. Finely chop the stems. Place the mushroom caps on the prepared sheet pan.
Heat a skillet over medium-high heat, until the pan is very hot, about 3 to 4 minutes. Add the diced stems and cook in the hot pan for 2 to 3 minutes, or until any excess moisture from the mushrooms has dried out. Reduce the heat to medium, add olive oil and onions to the pan and sauté with the mushrooms for 2 to 3 minutes. Season with salt and black pepper. Add the creamed spinach and stir well to combine. Take a spoonful of the spinach mixture and fill the cavity of each mushroom cap.
Combine panko, Parmesan cheese, and melted butter in a small bowl. Sprinkle the panko mixture on top of the stuffed mushrooms. Bake for 18 to 20 minutes or until golden brown.
Garnish mushrooms with fresh chives and serve immediately. Enjoy!
