Preheat oven to 400°F. Spray a baking sheet with olive oil spray. Spread the pierogies in a single layer on the baking sheet and spray both sides with olive oil spray.
Bake pierogies for 20 minutes, turning once halfway through the baking time. Remove and allow to cool slightly, enough to handle.
Meanwhile, melt butter in a 10-inch cast iron skillet set over medium heat. When butter starts to foam, add garlic and toast 30-45 seconds until fragrant. Transfer to a small bowl.
Arrange pierogies in a circle on the outside of the skillet, sprinkling a handful of shredded Monterey Jack cheese between each one. When all pierogies are in the skillet, drizzle with garlic butter, then top with mozzarella cheese.
Bake for 8 to 10 minutes until cheese is melted. Garnish with parsley and Parmesan if desired. Serve with marinara sauce.