Creamy chicken and wholegrain mustard and mushroom sauce

To accompany 2 leftover chicken legs we made Ottolenghi's sauce :

1 tbsp sunflower oil
Salt and freshly ground black pepper
15g unsalted butter
1 large onion, peeled and finely diced
Some thyme sprigs from the garden
200g mushrooms, halved
1 tsp paprikapowder
200ml chicken stock from our freezer
2 tsp wholegrain mustard
80g double cream
8 small gherkins, cut in half lengthways
Some parsley leaves, roughly chopped
Some tarragon leaves, roughly chopped (in the garden they were frozen, so we used dry ones from a pot)

Put the oil in  a large sauté pan on medium heat. Add the butter, onion and thyme, and saute for five minutes, until softened. Add the mushrooms, cook for eight minutes, until they take on a little colour, then add the paprika, half a teaspoon of salt and a good grind of pepper. Stir, cook for a minute, then add the stock and mustard. Cook for five or so minutes, to thicken the sauce, stir through the cream and cook, uncovered, for 10 minutes more; if the sauce needs further thickening, reduce the sauce. Off the heat, stir in the gherkins, parsley and tarragon, and serve piping hot – mashed potato or plain boiled rice would make a very happy partner, but we didn'tl.