If you want to brown the pierogis first, place the butter in a frying pan over medium heat. Add the pierogis to the melted butter and brown, approximately 3 minutes on each side.
Place the pierogis in a large bowl.
Mix in the sour cream, sausage, 1 chopped green onion and 1/2 cup of cheese.
Pour the pierogi mixture into a 9x9 casserole dish and top with remaining cheese.
Bake for 22 minutes until the cheese is completely melted.