Preheat the oven to 400°F (200°C).
In a large pot, melt 2 tbsp (30 ml) of the butter over medium heat. Add the leeks and season with salt. Cook, stirring, about 5 minutes or until softened. Add the carrots and garlic. Cook for 3 minutes, stirring occasionally. Sprinkle the flour over the vegetables and cook, stirring, for 1 minute.
Add the chicken, spinach, kale, stock, cream and mustard. Season with salt and pepper. Simmer gently until the sauce has thickened and the kale and spinach have wilted, about 5 minutes. Stir in the fresh herbs and adjust the seasoning.
Transfer the mixture to the Roaster. Cut the filo sheets into 4 squares. Melt the remaining butter. Lightly brush each square of filo with butter, scrunch into the shape of a flower and place over the chicken mixture. Continue until the entire surface of the pot pie is covered in filo flowers.
Bake for 20 minutes, until the filo is nicely golden. Let sit for 10 minutes before serving.