To Prepare Lemon Slices and Line Mold: Stir sugar and 1 ½ cups water together in a small saucepan. Bring to a simmer. Meanwhile, wash and slice lemons ⅛ inch thick; you will need about 18 good slices. Poach lemon slices gently in simmering syrup, covered, until tender, 15–20 minutes. Cool poached fruit in syrup. Cover and chill until needed, up to 2 weeks in advan