Preheat oven to 350 degrees. Cook spinach; drain and squeeze dry. Saute onion in butter until transparent. Combine sour cream, 1/2 cup Parmesan cheese, salt, pepper, and cooked spinach in a mixing bowl. Mix in onion. Pour into a 1 1/2 quart baking dish. Drain artichoke hearts, arrange over the spinach mixture. Sprinkle with remaining 2 tablespoons of Parmesan cheese. Bake for 20 - 30 minutes until hot and bubbly.
If you are preparing this ahead, you will need to increase the baking time if you are baking directly from the refrigerator. I always take the casserole out of the refrigerator and let it sit at room temperature approx. 30 minutes prior to baking.