Grate and soak the potatoes. Peel the potatoes and grate them on the large holes of a box grater. Immediately place the shreds into a large bowl of cold water with the lemon juice. Let them soak for 5 minutes—this washes away excess starch that would otherwise make the hash browns gluey. You'll notice the water turning milky white.

Squeeze out every drop of moisture. Drain the potatoes in a colander, then tip them onto a clean tea towel (or use several layers of kitchen paper). Gather the corners and twist firmly, wringing out as much liquid as you can. When you open the towel, the shreds should feel dry and separate easily, not clumped together. This is the single most important step for achieving crunch.

Season and rest the mixture. Transfer the dry potato shreds to a mixing bowl. Toss with the salt, pepper, onion powder (if using), and cornflour. Spread the mixture on a plate in a thin layer and let it sit at room temperature for 10 minutes. During this resting time, the salt draws out any remaining moisture—you'll see tiny beads of water form on the surface. Pat the shreds dry again with a paper towel before cooking.

Heat the oil until shimmering. Place a large non-stick skillet (about 30 cm) over medium-high heat. Add the oil and swirl to coat the base. When the oil shimmers and you see faint wisps of smoke rising, it's ready—this should take about 2 minutes. If you drop a shred of potato in and it sizzles immediately, you're good to go.

Form the hash browns in the pan. Carefully add the potato shreds to the hot oil, spreading them into an even layer about 1 cm thick. Don't press them down yet—let them fall naturally. You should hear a loud, steady sizzle as soon as the potatoes hit the oil. If the sizzle fades, the pan isn't hot enough.

Cook undisturbed for 5–6 minutes. Let the hash browns cook without moving them. After about 4 minutes, check the underside by gently lifting a corner with a spatula. You're looking for a deep golden-brown colour and a crispy, lacy edge. The surface of the uncooked side will start to look slightly translucent as the heat travels upward.

Flip and add the butter. Slide a large spatula under the hash browns and flip in one confident motion. Drop the butter into the pan around the edges, tilting the pan so the melted butter spreads underneath. Cook for another 4–5 minutes, until the second side is equally golden and the edges are shatter-crisp. The smell will shift from raw potato to a nutty, toasted aroma.

Drain and serve immediately. Transfer the hash browns to a plate lined with kitchen paper to absorb any excess oil. Let them rest for 1 minute before cutting into wedges or squares—this allows the crust to set fully. Serve hot, while the exterior is still audibly crunchy.