Is there a substitute for Seville oranges? In my country you never see them in the supermarkets or grocery shops.
Seville oranges are bitter oranges that originated in Asia but travelled westwards as trade routes opened up in the 10th century and since then the oranges have been widely cultivated in Spain. Their floral sharpness makes them useful in desserts, such as Nigella's Bitter Orange Tart, to balance out sweetness, but they can also be good for savoury dishes, such as Nigella's Chicken Traybake With Bitter Orange And Fennel. Seville oranges are also good for marmalade as they are high in pectin, which is the element that makes jams set.
Unfortunately for Seville orange marmalade there is no good substitute, though you can make marmalades with other citrus fruits. For the traybake Nigella gives an option in the ingredients list for a mixture of sweet oranges and a lemon. For desserts Nigella usually suggests a mixture of regular sweet orange juice and lime juice in a ratio of 2:1 (i.e twice the volume of orange juice to lime juice). The zest of orange and lime can also be used in combination if the recipe calls for grated zest.