- Preheat oven to 350°F (175°C). Prepare a large baking pan (16 x 12 inches – 40 x 30 cm) by spraying with vegetable oil cooking spray or coating with oil.
- Heat oil over medium-high heat in a skillet. Dice the onions and cut the chicken breast into bite-size pieces – about ½ inch (1.25 cm) diced. Add to the skillet and cook through and until lightly browned - about 10 minutes. Remove from heat and set aside.
- Spread 1 cup (250 mL) of sauce over bottom of baking pan. Top with the frozen pierogies arranging them flat side down.
- Spread the package of defrosted spinach evenly over top of pierogies. It isn’t necessary to wring the spinach out.
- Grate cheese. Mix in a bowl with herb seasoning. Sprinkle casserole with half the cheese.
- Sprinkle chicken and onion mixture evenly over.
- Pour over remaining 2 cups (500 mL) of Marinara sauce. Finish by sprinkling remaining cheese and herb mixture over top.
- Bake uncovered in preheated oven for 30-40 minutes or until heated through, pierogies are tender and cheese is lightly browned.
Pierogies are already cooked and don't have to be boiled before using.