The Chicken And Pea Traybake did not work for me since it yielded a tasteless and salty dish. The only substitution I made was fairly large-bore scallions (middle finger size) for the leeks. The result was underdone chicken, burned/blackened peas, scallions, and garlic, a far too oily "pan sauce" and too much salt.
Nigella's Chicken And Pea Traybake (from AT MY TABLE roasts chicken thighs on a bed of leeks and frozen peas. The peas braise in the vermouth and chicken juices and the chicken should cook through with a crisp skin and moist flesh. Nigella mentions in the introduction to the recipe that the size of the tin used is very important and from the description it could be that the tin was too small. The tin needs to be at least 28 x 38 cm (11 x 15 inches) as if it is smaller the chicken thighs will be too close together to allow the hot air to circulate around them and they will not cook and colour properly.
If the peas are burning you may need to add a little more liquid when you check the dish after the first 45 minutes of cooking. This could be a couple of splashes of water from a freshly boiled kettle rather than extra vermouth. The peas should be just submerged in liquid at this point. Salt can be a matter of personal taste but we would mention that the recipe gives measurements for sea salt flakes. If you are using fine sea salt or table salt then you need to reduce the quantities and use only 1 teaspoon.