Preheat oven to 350 degrees Fahrenheit. Spray a 9- or 10-inch quiche dish with non-stick cooking spray.
Boil pierogies as box directions and drain.
Melt butter in a 12-inch skillet over medium heat. Add red pepper and mushrooms. Cook, stirring frequently, about 5 minutes, or until just tender. Remove to bowl with a slotted spoon.
Add spinach and scallions to drippings remaining in the skillet. Cook for about 3 minutes or until wilted. Remove to bowl with vegetables.
In a large bowl, beat milk, eggs, salt, and pepper until well-mixed. Add cooked pierogies, vegetables, and cheese.
Pour mixture into prepared quiche dish. Bake 38 to 40 minutes or until mixture is set and golden. Serve Pierogy Quiche and enjoy!