Preheat an oven to 375 F. Coat a 9 x 9 baking dish with nonstick cooking spray, set aside.
Nonstick cooking spray
In a large skillet over medium-high heat, saute the onion and garlic in the olive oil until translucent, about 7 to 8 minutes.
2 Tablespoons olive oil, 1/2 onion, 2 cloves garlic
Remove from heat, then stir in the cream of mushroom soup, sour cream, milk, cheddar cheese and bacon.
1 can, 1/2 cup sour cream, 1/4 cup milk, 1 cup cheddar cheese, 1/2 cup bacon
Spread half of the cheese mixture along the bottom of the baking dish. Arrange the pierogies over the top, shingling them to fit if necessary.
1 package
Spread the remaining cheese mixture over the top, making sure all the pierogies are coated.
Sprinkle the chives over the top. Bake for 30 for 40 minutes until the top is golden brown and the sauce is bubbling around the sides of the dish. Let cool for 4 to 5 minutes before serving.
3 Tablespoons fresh chives