Preheat the oven to 350°F. Place a baking rack on a rimmed baking sheet; set aside.
Heat the olive oil in a small sauté pan over medium-high heat. Add the chopped mushroom stems and onion. Cook, stirring frequently for 3 minutes, until softened.
In a medium bowl, combine the mozzarella cheese, artichokes, cream cheese, Parmesan cheese, garlic salt, and ground black pepper. Stir in the cooked mushroom stems and onion. Adjust seasoning to taste.
Fill the mushroom caps with generous spoonfuls of filling and set them on the baking rack. Bake for 20 minutes, until heated through. Sprinkle with ground cayenne pepper, if using, and serve.