Using a cloth or paper towels wrap the grated potato up and squeeze out all of the liquid (potato starch).

In a large bowl combine grated potato, egg, cornstarch, salt and pepper. Mix until combined.

Heat a pan (cast iron is best) over medium-high heat, add 1 tablespoon oil and wait for it to heat up.

Add half of potato mixture to pan and flatten it to make sure it's even all around. Reduce heat to medium.

Cook for 8-10 minutes (checking at 6 minutes) on one side or until golden brown, and cook 6-8 minutes (adding more oil if necessary) on the other side (to flip rosti over - slide the rosti onto a plate first, then cover it with another plate, flip the two plates and then slide the uncooked side back onto the pan). Repeat with the other rosti.

For the mushroom sauce heat butter in a pan over medium heat and add diced onion. Cook for a few minutes, remove, and add sliced mushrooms to the pan. Cook mushrooms until starting to brown, add onions back to the pan, reduce heat to low, and add a few dollops of sour cream (to reach desired consistency).

Serve rosti with mushroom sauce and fresh slivered basil (optional).