I love Nigella's Spanish Chicken traybake recipe (from Kitchen) using chicken thighs, new potatoes and chorizo and have cooked it lots of times. What could I serve it with vegetable-wise that would really complement the flavours?
A traybake is designed to be a fairly low maintenance dish, as all of the ingredients are cooked at the same time in the oven, requiring merely a quick baste and rotation of baking sheets halfway through the cooking time. Consequently the accompaniment suggested by Nigella is no more than a large, plain green salad that will act as a little colour and texture on the plate and will let the flavours of the dish shine through.
However if you wish to add a side dish with more substance then we would suggest considering something like Pisto Manchego which is a stew of vegetables and tomatoes which is very similar to the French ratatouille. As the dish is cooked with tomatoes it can also act as a sort of sauce for the chicken and chorizo. There are several recipes available on-line.
The Spanish will also often serve vegetables as a first course and then just have a main (entree) course with potatoes and meat. If you would like to go down this route then we would suggest trying the Catalan dish of sauteed swiss chard or spinach with raisins and pine nuts. It is often served with slices of toasted country bread to make it a fairly substantial appetizer.