I've had a problem where the onions are burning when I make the Spanish Chicken. Just curious as to why they burn?
Nigella's Spanish Chicken With Chorizo And Potatoes (from KITCHEN) is a popular all-in-one traybake. The potatoes and onions are flavoured with the spiced oil from chorizo sausage and the onions should cook until soft and slightly caramelized at the edges. As a fair amount of oil is rendered from the chorizo, it is a good idea to use a shallow roasting pan or a baking sheet with an extra deep lip.
If the onions are catching slightly, there could be a few reasons. They are most likely to catch if the ingredients are quite spread out in the roasting pan. Try to use a pan that holds all of the ingredients snugly. The onions should also be only very roughly chopped and it may help to cut them lengthways into slices with a little of the root attached, so that they hold together rather than separating out into smaller, individual pieces (which are more likely to burn). Also, if the lip of the baking sheet is too shallow, the onions will get more direct heat, causing them to cook more quickly. If your pan does have a shallow lip then it may help to put the onions towards the centre of the pan and sit the chicken thighs on top of them, to provide a little extra protection.