Preheat oven to 425°F (218°C). Fill the bottom of a large microwave-safe bowl with 1/2-inch of water. Put the potatoes in it and cover with plastic wrap. Poke the wrap a few times with a fork.
Microwave on high heat for 10 minutes until the potatoes are cooked and can be easily pierced through with a fork. Microwave for a few more minutes if they are still hard.
Remove and uncover the wrap. Drain and cool the potatoes until just warm. Pour the oil onto a large rimmed baking sheet and sprinkle the oil with 3/4 teaspoon of the salt and 1/2 teaspoon of the pepper.
Arrange the potatoes on the baking sheet and smash them into rounds using flat-bottom cup or the heel of your hand. Sprinkle the top of the potatoes with the remaining salt and pepper.
Put the baking sheet on the floor of the oven (bottom part and not on the rack) and roast until the undersides are crispy but not burnt, about 12-15 minutes. Remove the baking sheet from the oven and use a flat spatula or turner to flip the potatoes.
Roast for another 10-15 minutes until the other sides are crispy. Remove from the oven, transfer the smashed potatoes to a serving platter, garnish with chopped parsley and serve immediately.