Slice the sweet potatoes to give you 50 x 1cm-thick slices. Spread out evenly on one of the lined trays and brush with oil on both sides using 2 tablespoons of the olive oil. Roast in the oven for 40 to 45 minutes, until cooked through and golden. Turn them over halfway through for even cooking and move the ones from the outside of the tray to the inside, as they will cook faster. Allow to cool.