Preheat the oven to 425°F (218°C). Fill the bottom of a large microwave-safe bowl with 1/2 inch (1 cm) of water. Place the potatoes in the bowl and cover with plastic wrap, poking the wrap a few times with a fork.
Microwave on high for 10 minutes, or until the potatoes are cooked and can be easily pierced with a fork. Microwave for a few more minutes if they are still hard.
Remove the plastic wrap and uncover the bowl. Drain the potatoes and let them cool until just warm. Pour the oil onto a large rimmed baking sheet and sprinkle it with 3/4 teaspoon of salt and 1/2 teaspoon of pepper.
Arrange the potatoes on the baking sheet and smash them into rounds using a flat-bottomed cup or the heel of your hand. Sprinkle the tops of the potatoes with the remaining salt and pepper.
Place the baking sheet on the bottom of the oven (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.