Heat oven to 350°F.
In a large skillet, melt butter over medium heat. Add onion and cook for about 8 minutes, stirring occasionally, until golden. Add garlic and cook for about 1 minute. Stir in spinach, artichokes, cream cheese, sour cream, 1 cup of the cheddar cheese, the Parmesan, paprika, pepper, salt and red pepper flakes. Stir well.
Place dip in an oven-proof 2-quart dish and top with remaining 1/2 cup of cheddar cheese. Bake at 350°F for 10 minutes or until warmed through. Serve with chips or crackers on the side.