Preheat oven to 375°F Generously coat a baking sheet with the cooking spray; set aside.
In a bowl, combine the potatoes, matzo meal, potato starch, onion, egg whites, pepper, and salt; knead together. Divide the dough into 6 balls and flatten each. Divide the broccoli evenly onto each circle, fold over, and press edges to seal.
Arrange the knishes in a single layer and place the baking sheet on the bottom rack of the oven. Bake for 15 minutes on each side. Serve hot.