Place the prepared potatoes in a microwave dish and cook on high for around 5 minutes. You do not want the potatoes overcooked and mushy, they should still have a slight bite in the center.

While the potatoes are cooking, heat 1 Tb olive oil in a heavy 10-inch skillet and add the onion and mushrooms. Cook the mushrooms and onions until soft and translucent and just beginning to brown, around 6 - 8 minutes. Add the minced garlic, thyme and a pinch of salt and pepper, stir to mix and cook until you begin to smell the garlic's perfume. Remove the mushrooms and onion from the pan and set aside.

Wipe the skillet clean and return it to the burner. Turn the heat to medium and add 2 Tbs to the skillet. Heat the oil until shimmering. Make sure there is an even coating of oil across the whole pan, then spoon half of the potatoes into the skillet. Press down on the potatoes with a rubber spatula and form the potatoes into a pancake. Season lightly with salt and pepper, then spread the mushrooms and onions over the potatoes. Add the remaining potatoes to cover the mushrooms and onions, then press down on the potatoes to cover the top of the pancake.

Cook the rösti on one side for around 7 minutes. Do not disturb the pancake for at least 4 minutes into the cooking time. After 7- 8 minutes, run a thin spatula around the edges and underneath the potatoes to loosen it from the bottom.

Slide the potatoes onto a plate large enough to hold the rösti. Place another plate, upside down, on top of the plate holding the rosti, so the rims are kissing each other. Flip the plates over, so the bottom plate is now the top and lift off the plate. You should see a beautiful golden brown crusty rösti.

Wipe off any stuck bits from the bottom of the pan and add 2 Tbs olive oil.

When the oil is shimmering, slide the rösti back into the skillet and sprinkle with salt and ground pepper. Cook for 7 more minutes.

When finished, loosen the rosti from the pan and slide it onto a serving plate.

Keep warm or serve immediately.