In a large pot of salted boiling water, cook the potatoes until fork tender, about 15-20 minutes. Drain and allow potatoes to cool slightly.
Transfer the potatoes to a large rimmed baking sheet. Use a potato masher or the bottom of a measuring cup or glass to press down on the potatoes to smash them.
Drizzle the olive oil on top of the potatoes, add the garlic, rosemary, parmesan cheese, salt and pepper, and toss with your hands to combine.
Bake for 15 minutes until the bottoms are golden. Use a spatula to flip them and add more salt, pepper and olive oil, if desired. Return to the oven for 12-15 more minutes until the potatoes become crispy.
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Notes
Storage: Store any leftovers in an airtight container. They will last about 5 days in the fridge. To reheat, just place in the oven at 350°F for 5-7 minutes or in a toaster oven. You can also reheat in the microwave, but the potatoes won't retain a crispy texture that way.Substitutes: For best results, follow the recipe as is. However you can switch out the herbs if you'd like, use butter instead of olive oil and make these with other types of potatoes.