Heat 1 tbsp olive oil in a large pan over a medium heat and lightly brown the chicken thighs for a few minutes on each side, until golden. Add the mushrooms and continue to cook for 4 minutes.
Add the leeks and cook until just softened, constantly turning the ingredients in the pan to allow everything to cook evenly. Add the gravy, crème fraîche and mustard, bring to the boil and season with salt and pepper.
Pour the contents of the pan into a large, shallow pie dish (20cm x 30cm).
Toss the potato, swede and parsnip in a bowl with 1 tbsp oil and season with salt. Place in a microwaveable dish and heat in the microwave at full power for 5 minutes to soften.
Layer the sliced veg over the pie mixture. Sprinkle with cracked black pepper, cover with foil and bake for 25 minutes. Remove the foil and return to the oven for another 10 minutes, or until the root veg topping is soft.