Preheat your oven to 200°C (180°C fan/gas mark 6).
Lightly season the chicken breasts with salt and pepper, and wrap them in foil to keep them moist. Bake in the oven for 20-25 minutes, or until the chicken is cooked through. Once cooked, allow the chicken to cool slightly, then dice or shred.
Boil the pasta in salted water according to the packet instructions until al dente. Drain and rinse under cold water to cool quickly. Set to one side.
In a small bowl, whisk together the fat-free natural yoghurt, light mayonnaise, Dijon mustard, and lemon juice. Season with salt and pepper to taste. Set to one side.
Once the chicken and pasta are all cooled. In a large bowl, combine the diced or shredded chicken, defrosted peas, and sliced spring onions.
Add the cooled pasta to the bowl with the chicken, peas, and spring onions. Pour over the dressing and mix until everything is evenly coated.
Cover and refrigerate for at least 30 minutes before serving to allow the flavours to meld together.