Add the sunflower oil to a medium-size roasting tray and place this in the oven at 200 °C/ 400 °F.
Wash and peel the potatoes. Slice any particularly large potatoes into halves - you are aiming for golf-ball sized pieces.
Add the potatoes to a large pot and cover with boiling water. Add the bicarbonate of soda and 1/2 tsp salt and stir. Bring to a boil over high heat, then cover and simmer the potatoes on low heat for 5 minutes.
Drain the potatoes into a colander and swish the potatoes around the colander until the edges are rough. Don't skip this step - it's what makes our potatoes extra crispy!
Remove the roasting tray from the oven and carefully add in the potatoes - they will splatter so I recommend adding the potatoes to the tray one at a time.
Add the rosemary sprigs and a generous pinch of sea salt. Toss the potatoes to coat them in the oil and seasonings.
Return the tray to the oven and cook the potatoes for at least 30 minutes, tossing the potatoes halfway through cooking. If you like your potatoes EXTRA crisp, cook them for an additional 5-10 minutes.
Serve immediately and enjoy!