Mix together the Alfredo sauce, milk, 1 1/2 cups of the Cheddar cheese and 1/4 cup of the Parmesan together in a saucepan.
Heat over low to medium heat, stirring until the cheese melts.
Cover the bottom of a greased 9x13 inch casserole dish with a couple of spoonfuls of the sauce, and then layer over that one box of the pierogies.
Next layer half of the sauce and all of the partially cooked broccoli (if desired, or you can leave this out).
Layer on the other box of pierogies.
Cover with the remaining sauce Cheddar and Parmesan cheeses.
Cover with foil and bake at 350 degrees for 20 minutes, then remove the foil and bake for another 25 minutes.