Pre heat oven to 200C / 400F / Gas 6
Heat oil in a pan, over a high heat, and brown the chicken thighs. Remove from the pan and set aside
Reduce to a low heat, add the onions and garlic and soften them in the chicken juices. (For around 5 mins) Remove from the pan and set aside with the chicken.
Using the same pan melt 40g of the butter and add the flour and mustard powder to form a roux. Slowly add the stock a little at a time, stirring continuously, until the sauce is smooth. Simmer for 3 mins and then add the herbs.
Put the chicken and onions back into the pan with the sauce, add the cream and stir.
Pour the mixture into an ovenproof dish (my dish was 10" X 7.5") and stir through the peas.
Peel and then thinly slice the potatoes (approx 5mm). Arrange the slices over the chicken mixture (overlapping so the surface is completely covered.) SEE NOTE 5
Melt the remaining butter and brush over the potatoes
Bake for 35 mins until the potatoes are cooked and crispy.