Peel and finely chop 7 of the oranges, discarding the pips. Reserve the skin of 3, discarding the rest, and finely chop into fine strips. Halve the remaining orange, skin-on, and finely slice. Add this to the pan with the cardamom seeds and 400ml water. Add the vanilla seeds and throw in the pod.
Boil for 10 mins until the skins are softening, then add the lemon juice and sugar, stirring constantly. Once the sugar has dissolved, simmer for 30-35 mins. Turn up the heat and boil to set (about 10-15 mins) – the boiling point of jam is 105C. If you don’t have a cooking thermometer, try the ‘wrinkle test’ – spoon some marmalade onto a cold saucer and leave for a minute. If it wrinkles when you poke it and has a fine skin on top, it’s ready to set. Pour the marmalade into sterilized jars, top with wax discs and lids, and store for up to a year.