Wash, brush, and dry the potatoes.
Slice each potato thinly (2–3 mm), making sure not to cut all the way through. Use wooden spoons or chopsticks on either side of the potato to act as a guide.
Grease a steel roasting pan with olive oil and place the potatoes in it, sliced side up.
Mix the softened butter with crushed garlic, chopped rosemary and thyme, salt, and black pepper.
Spread half of the butter mixture between the slices and over the top of the potatoes.
Preheat the grill to 250°C. Set side burners to maximum and middle burners to medium, then turn off the middle burners and place the roasting pan over them. Close the grill lid and cook for 30 minutes.
After 30 minutes, brush the potatoes with the remaining butter and sprinkle with additional salt.
Turn on the middle burners to minimum heat and grill for another 20 minutes. Maintain the grill temperature between 250–280°C; reduce external burner power if it gets too hot.
For larger potatoes (over 100g), increase cooking time or cover with aluminum foil. Alternatively, finish them on the top shelf of the grill.
