Heat extra virgin olive oil and ½ ounce (14 grams) rosemary in a saucepan over medium high heat until it reaches 230°F (110°C) on an instant read thermometer. Remove from heat, and allow the oil to cool until it reaches 120°F (50°C) on an instant read thermometer. Once cooled, remove and discard the rosemary.
Add in an additional ½ ounce (14 grams) rosemary. Repeat the heating and cooling process two additional times, removing and adding the remaining ½ ounce (14 grams) rosemary for the last batch.
Allow oil to cool to room temperature. Strain and discard rosemary.
Store in a sterilized dark-colored bottle with a sealed lid (you can reuse the bottle the oil came in).
Heat oven to 425°F (220°C). Line a large baking sheet with parchment paper.
Toss sliced potatoes, 3 tablespoons (45 ml) rosemary infused olive oil, salt, and pepper together in a large mixing bowl. Arrange potatoes on prepared baking sheet.
Roast potatoes for 38–45 minutes, flipping halfway through the cooking process, until the potatoes are golden brown and fork tender.
To serve, optionally garnish with additional finely chopped rosemary.