Slice the sweet potatoes as thinly as you possibly can (the use of a mandoline is strongly recommended) and arrange the slices vertically in a 10" round baking dish
Drop little dabs of butter or ghee on top of the potatoes, then sprinkle the chopped pecans, green onion, fresh rosemary, salt and pepper. Drizzle the maple syrup all over the potatoes, cover with aluminium foil and bake for 50 minutes, or until the potatoes are tender.
Remove the foil and continue baking for 15 to 20 minutes, until the top starts to brown and crisp up a little.
If you find that the top hasn't browned enough for your liking, set the oven to broil for 2 to 3 minutes, but keep a close eye on things so you don't end up burning the pecans.