Wash the potatoes. Place the first potato on the cutting board. If you’ve never cut hasselback potatoes before, you can use two wooden spoons to make the process easier. Place the potato between the handles of the spoons so that when you slice down into the potato, the spoons stop the blade from going all the way through. Carefully slice the potato very thinly. Repeat with all potatoes.
Place all the potatoes on a baking paper on baking tray.
Melt about 100 g of the butter. Spoon it over the potatoes. Sprinkle the thyme leaves and salt over too.
Cut the rest of the butter into thin slices and place on the potatoes.
Roast the potatoes in the oven for 30 minutes, add the crumbled feta and keep roasting for about 30-40 more minutes. The potatoes should be soft inside and crispy on top. Enjoy with other vegetables or for example with pan fried fish!