To a large bowl, add the potatoes, onion, salt, pepper, and toss with a fork to combine.
Evenly brush the grates of the waffle iron with melted butter.
Add the potatoes to iron, evenly mounded in the center, and lightly press iron to close it, noting that it may not close completely right away. Cook for 2 minutes.
Close iron completely and cook for about 12 to 20 minutes, or until crisp and golden brown. Peek at about 10 minutes to see how it looks. Cooking time will vary based on how hot your waffle iron gets, how much moisture is in the potatoes used, personal preferences for crispiness, etc.
Optionally add sour cream and chives before serving. Hash browns are best warm and fresh.
Tip – midway through cooking if you want to accelerate the process, brush additional melted butter onto the top grates of the iron or directly on the potatoes.