Heat a dual-contact indoor grill such as a George Foreman grill. Or heat a double-burner grill pan over medium-high heat.
Wash and peel the potatoes and onion. Shred in your food processor using the shredding attachment. Mix together and place in a colander to drain. Press on them with your hands to extract as much water as possible, then extract even more moisture using paper towels. The drier the mixture is, the crispier the hash browns will turn out.
Brush the grill surfaces with oil. Avocado oil is a healthy oil that can withstand high temperatures, so it's a good choice.
Place the potato mixture on the grill, spreading it evenly, then pressing on it to flatten. Sprinkle with salt and pepper.
Close the lid (if using a George Foreman grill) and grill for 10-15 minutes.
Use a large spatula to transfer the partially cooked hash browns to a cutting board. Brush the grill again with more oil, flip the hash browns to the other side and grill 10-15 more minutes, until browned and crispy on both sides.