- Preheat oven to 220C.
- Butterfly the chicken by removing the backbone then place chicken, breast side up, into a shallow tray. Press down on the breastbone firmly to flatten the chicken out. Drizzle with oil and season with salt and pepper.
- Place into the oven until cooked through, approximately 55-60 minutes. Remove from the oven and allow to cool. Remove the meat from the bones; shred the meat then place into a medium bowl.
- Finely chop the walnuts, celery and chives and add to the chicken. Add the crème fraiche, lemon zest and lemon juice. Toss to combine and season well to taste.
- Butter each slice of bread. Add a generous amount of filling to half of the slices and top with another slice. Cut each sandwich in half to serve.