Preheat oven to 375 degrees. Wash and dry mushrooms. Pull the stems off the mushrooms, then chop the stems finely. Heat olive oil and butter in a skillet over medium-high heat. Add onion and chopped mushrooms stems and saute for several minutes, or until soft and starting to turn golden. Add panko crumbs, salt, and pepper. Stir and continue cooking for 2 minutes. Remove from heat and set aside to cool. Squeeze excess liquid out of thawed spinach. Set aside. In a large bowl, add softened cream cheese, sour cream, grated cheeses, and the cooled mushroom/onion/panko mixture. Fold together to combine. Add spinach, a dash of salt, a little pepper, and hot sauce to taste. Fold together until combined. Using a spoon, mound the mixture into the stemmed mushroom caps. Use the spoon to round the mixture on top. Place on a baking sheet or in a baking dish and bake for 20 to 25 minutes, or until the mushrooms are cooked and the filling is hot. Serve immediately or at room temperature. Yummy!