This is one of my most prized recipes. It’s easy, make-ahead, makes a huge amount (if you have a big gift list), and makes great use of those seasonal holiday boxes of clementines. It’s a wonderful tart but a not-bitter marmalade. Tuck it in a basket with some English muffins for a cosy winter gift. Thin cut clementines is imperative for this preserve – a mandolin works best.