Chop onion and spinach into small, thin slices. In a large skillet, melt 2-3 tablespoons of butter and then add onion. Cook onion until clear. Add the chopped spinach to the onion and let it slowly wilt with heat. Add the artichokes. Add the cream cheese, sour cream, Parmesan cheese into the skillet and slowly mix until everything is blended together. Add pepper flakes to the mix and stir gently. Once the mix has started to bubble, turn off the heat and pour into a bowl. Add a thin layer of pepper jack cheese on top and serve.