Mix the panko, Parmesan, onion powder, cayenne and 1/2 teaspoon salt together in a wide, shallow bowl. Whisk the eggs and milk together in another shallow bowl.
Toss the green beans in the flour. Working a few green beans at a time, shake off the excess flour and dip in the egg. Let the excess egg drip off and then toss in the panko, patting firmly so the crumbs adhere. Arrange the breaded green beans in a single layer in the prepared baking pan and spray thoroughly with cooking spray.
Bake until golden brown and crisp-tender, about 10 minutes.