Set up your station. Grab a large bowl, a box grater with big holes, a clean kitchen towel, and a 10 to 12 inch skillet. Cast iron is ideal, nonstick works too.
Scrub or peel the potatoes. I usually peel for that classic diner look, but skins add nice texture. Your call.
Shred the potatoes on the large holes of the grater. Work quickly so they don’t oxidize too much.
Rinse the shreds. Put them in the bowl and cover with very cold water. Swish with your hands for 10 to 15 seconds until the water goes cloudy. Drain and repeat with fresh cold water until the water runs mostly clear. This step removes excess surface starch that can make hashbrowns gummy.
Soak briefly. Let the clean shreds sit in fresh cold water for 5 to 10 minutes. This helps keep them from browning too fast and releases a bit more starch.
Drain very well. Scoop the shreds into the kitchen towel and wring them out like you mean it. Twist and squeeze until no more water drips. Drier shreds equal crispier hashbrowns.
Season lightly. Toss the dry shreds with the salt and pepper. If using onion or spices, mix them in now.
Preheat the skillet over medium to medium high heat for 2 to 3 minutes. Add the butter and oil. When the fat shimmers and a strand of potato sizzles on contact, you’re ready.
Spread the potatoes. Add the shreds in an even layer about 1/4 inch thick. Don’t pile them high. Press gently with a spatula to compact them, especially around the edges. I like to shape a neat round, but rustic works too.
Let them be. Cook without moving for 5 to 7 minutes, until the bottom is deeply golden. You’ll hear a steady soft sizzle. If it sounds angry, lower the heat. If it’s whisper quiet, nudge it up. A little patience goes a long way here.
Edge insurance. Drizzle a teaspoon more fat around the edges if the pan looks dry. It creeps under and helps that crispy lace form.
Flip in sections. Slide your spatula under one quadrant and flip, then repeat with the rest. If you’re brave, flip the whole thing, but no heroics needed. Cook the second side 3 to 5 minutes until golden and crisp.
Taste and adjust. Sprinkle a pinch more salt if needed. Slide onto a plate and serve immediately. Hot sauce, ketchup, or a fried egg on top. Up to you.