Preheat your oven to 200°C (180°C fan) or gas mark 6. In a large, shallow casserole dish, heat the olive oil over medium heat. Sauté the spring onions for approximately three minutes until slightly softened. Stir in the frozen spinach and cook for an additional two minutes or until it begins to wilt.
Discard the chicken bones and shred the meat into the pan. Stir in the chicken stock and mustard, mixing thoroughly. Allow the mixture to simmer, uncovered, for 5–10 minutes to let the flavors meld together.
Incorporate the peas, crème fraîche, and freshly chopped herbs into the pan, stirring gently to combine. Remove from the heat. Layer the filo pastry sheets over the filling, scrunching them lightly for texture. Brush the pastry with a thin layer of olive oil.
Transfer the casserole dish to the oven and bake for 15–20 minutes, or until the filo pastry turns golden brown and crisp. Serve immediately and enjoy.