Preheat an oven to 180 degrees C. Grease a baking tray.
Heat the olive oil in a frypan over medium heat; cook the onions and mushroom stems in the hot oil until the onion is translucent; about 5 minutes. Season with salt and pepper.
Transfer the mixture to a large bowl then add the artichoke hearts, cream cheese, sour cream and mixed grated cheese. Season with salt, pepper and garlic salt. Stir the mixture until the ingredients are evenly distributed. Stuff the mushroom caps with the mixture. Arrange the stuffed mushrooms on the prepared baking tray.
Bake in the preheated oven until the filling begins to bubble, about 20 minutes.