Fill a large stock pot with water 3/4 the way. Add potatoes (making sure they are fully covered with at least an inch of water above them) and bring to a boil.
Cook on high for 20-25 minutes, until penetrable with a fork (larger potatoes will take longer to cook).
Once 5 minutes are left to boil, preheat the oven to 400°F and line a large baking sheet with parchment paper.
Mix together the oil, rosemary, minced garlic and onion powder in a small bowl.
Place the potatoes on the prepared sheet and smash. You can do this with a fork, bottom of a drinking glass or potato smasher. Brush each potato with oil mixture. Set aside remaining oil for later.
Place in preheated oven and cook for 15 minutes.
After the 15 minutes, take potatoes out and sprinkle evenly with 1/2 tsp salt.
Flip the potatoes carefully and brush with leftover oil (if there isn't any left, spray potatoes with cooking oil) and bake for another 10-15 minutes, until nice and crispy (broil on high for a minute or two if desired).
Top with the remaining 1/2 tsp salt and black pepper and serve (you can add more to taste).
