Heat vegetable oil in a large, high-sided frying pan over a medium-high heat. Sear chicken until lightly browned, stirring occasionally, for roughly 3 minutes. It does not need to be cooked through yet. Transfer to a plate.

In the same pan, reduce the heat to medium and melt the butter. Once melted, sauté the onion until just soft, about 4 minutes. Then add mushrooms and cook for 3 additional minutes.

Once the mushrooms are slightly softened, add the flour, mustard and thyme. Using a wooden spoon, stir the flour into the butter and mushrooms until it is incorporated and there are no visible clumps, approx. 2 minutes.

Gradually pour in the chicken stock, continuing to stir so the flour mixture combines. Bring to the boil, then reduce to a simmer, add the reserved chicken and cover. Simmer for about 15 minutes, or until the chicken is cooked.

With the heat on low, add the cream and season to taste with salt and pepper. Let the casserole cook for 2-3 more minutes so the flavours combine.

Serve with a sprinkle of fresh parsley and a wedge of lemon.