Preheat an oven to 200°C. Then heat the oil in sauté pan. Season the chicken with salt and pepper and brown in the oil, do this in two batches. Set the chicken aside and add the leeks and thyme to the pan, sauté until very soft, about 5 minutes, then set aside with the chicken. In a saucepan melt 50g butter, add ¼ cup of flour and cook for a minute or two until golden. Add the cream and whisk thoroughly to ensure there are no lumps. Add the chicken stock, stirring thoroughly until incorporated. Allow the sauce to simmer for 10 minutes then add the mustard and chopped parsley. Taste and adjust seasoning if necessary.
While the sauce is cooking, sift the remaining 1½ cups of flour, baking powder, 1/2 tsp salt, and baking soda into a bowl. Place in a food processor and combine with 75g chilled butter, cut into cubes,
to a coarse crumb. Return to the bowl and mix through the herbs. Quickly stir through the buttermilk to make the dough wet and sticky.
When the sauce is done add the chicken and leeks and pour into a large ovenproof serving dish. Place spoonfuls of the dough on top of the chicken filling and sprinkle with the Parmesan. Bake immediately for 30 to 40 minutes until the crust is golden and the filling is bubbling.