Slice the potatoes lengthways with a serrated knife (no more than 1/2 a thumb thick) and place in a bag. Mix together the spices and olive oil. Season with black pepper from the mill, then pour the marinade over the potatoes.
Close the bag and shake well until the potato slices are covered all around. Let it steep in the fridge overnight. Shake well again in between.
Roast the potato steaks on the grill until golden on both sides, brush with excess marinade. Before consumption, add a pinch of fleur de sel on top and serve.