Preheat the oven to 375 degrees F. Coat a 3-quart baking dish with cooking spray.
Place the bacon in a pan over medium high heat. Cook for 4-5 minutes or until bacon is crisp.
Remove the bacon from the pan and drain on paper towels.
Pour off most of the bacon grease in the pan; leaving 2 teaspoons. Reserve the extra bacon grease for later use.
Place the onion in the pan with the bacon grease and cook for 3-4 minutes or until translucent. Add the garlic and cook for 30 seconds.
Remove the onion and garlic from the pan. Add 2 more teaspoons of bacon grease to the pan.
Add half the pierogies to the pan, cook for 3-4 minutes per side or until browned.
Repeat the process with the remaining pierogies. Place all the pierogies, 3/4 of the bacon, onions and garlic in the pan and stir to combine.
Spread 1/3 cup of the Alfredo sauce over the bottom of the baking dish. Add 1/2 the pierogi mixture over the sauce.
Add another 1/3 cup of the sauce, then use the remaining pierogi mixture to create another layer.
Spread the remaining Alfredo sauce over the top of the pierogi mixture, then top the casserole with shredded cheese.
Bake for 15 minutes or until cheese has melted and started to brown. Top with reserved bacon and green onions, then serve.