Using a drier, starchier potato such as Idaho, and rinsing away the excess starch is a sure-fire way to crispy, golden hash browns. Butter helps both browning and flavor, while oil lowers the smoke point, allowing your pan to do the steady, even cooking needed for perfectly crisped potatoes.
2 medium idaho potatoes, peeled and shredded using the large holes of a box grater or grating disk of a food processor (about 2 cups)
½ small yellow onion, thinly sliced
2 tablespoons butter, divided
1 tablespoon grapeseed or canola oil
3 tablespoons shredded parmesan (optional)
Kosher salt and freshly ground black pepper, to taste
Rinse shredded potatoes in a colander under cold water until water runs clear, 3–4 times. Gently squeeze out all excess water and press potatoes between cloth or paper towels to dry completely. Toss potatoes with onion and season liberally with salt and pepper.
Remove the skillet from the oven and add 1 tablespoon each of butter and oil over medium heat. Once butter has melted, add potatoes and onions in an even layer, pressing lightly. Cook without disturbing until edges are deep golden brown, 10–12 minutes.
Remove pan from heat and place a large heatproof plate over the skillet. Carefully invert the pan to transfer the hashbrowns to the plate. Return the pan to medium heat and add remaining butter. Slide the hashbrowns back into the skillet browned side up and continue to cook until deep golden brown on the other side, 10–12 minutes more.
Sprinkle the parmesan over the hash browns (if using) and transfer to the oven. Bake until cheese is melted and bubbling and hashbrowns are deep brown and crispy, 5–7 minutes more.