Preheat oven to 375 degrees. Coat a round baking dish with cooking spray.
Thinly slice the potato. (Try using the prongs of a fork as a guide!). Dice the onion. (I like to finely dice mine so the onion flavor is throughout the dish, but I won't be biting into a large chunk of onion.)
Drain the spinach and squeeze as much of the moisture out with a towel as you can.
Layer half of the potatoes in the baking dish. Top with half of the chopped spinach, half of the onion, and half of the cheese. Repeat with the rest of the potatoes, spinach, onion, and cheese.
In a medium bowl, whisk together the eggs, milk, salt, and pepper. (My family likes pepper, so my typical "salt/pepper to taste" ratio is one pinch salt and two pinches of pepper.). Pour the egg mixture into the baking dish and sprinkle the rest of the cheese on top.
Bake for 45-60 minutes. Cut into wedges and serve hot or warm.