1. In a medium frying pan, saute onion and sausage over medium heat until onion is opaque. 2. Melt butter in a sauce pan over medium heat, add flour whisking constantly until combined. 3. Add cream to the sauce pan, mix well and bring to a boil stirring frequently. Remove from heat when the mixture begins to boil and thicken. Add salt and pepper to taste. 4. Place perogies in a 9x11" baking dish, from frozen. Top with onion, sausage and pour cream sauce overtop. 5. Sprinkle entire dish with shredded cheese and bake at 350 for 30 mins. 6. Top with green onions and serve.