Preheat oven to 400°F/205°C.
Cut 3mm-wide slices all the way along each potato, cutting about ⅔ of the way through with each slice. (To avoid cutting right through the potato, place handles of two wooden spoons on a chopping board on either side of it).
Chop sage very finely with a knife and place in a small bowl (if using dried, use a pestle and mortar to grind sage finely).
Add oil to sage and mix thoroughly.
Place potatoes on a baking sheet and using hands, rub sage sage/olive oil mixture into them.
Season with salt and pepper and bake in oven for 35 minutes.
Remove potatoes from oven and sprinkle crushed garlic over them, pushing it between slices so potatoes absorb flavour.
Using pastry brush, quickly re-coat potatoes in oil (there will likely be some in the bottom of the pan) and bake for another 20 minutes or until potatoes are golden and tender on the inside (test with a fork to ensure potatoes are cooked).
Remove and serve hasselback potatoes as they are, or with extra salt and freshly chopped herbs sprinkled over them for extra colour.