Preheat the oven to 425℉. Line a large baking dish with foil, and spray with nonstick spray.
In a large skillet over medium-heat, heat the oil and add the onion; cook and stir for about 5 minutes. Stir in the salt and pepper. Transfer the mixture to a large bowl. Stir in the artichoke hearts, cream cheese, Parmesan cheese, sour cream and garlic salt.
Stuff the mushroom caps with the artichoke mixture; arrange in the prepared baking pan. Bake until the mushrooms are tender and the filling is hot and bubbly, 17 to 20 minutes. Serve with a sprinkle of fresh parsley on top.
Notes
Make ahead tip: The mushroom caps can be stuffed and chilled up to one day ahead. Keep covered in the fridge.