Combine porcini, thyme, salt and pepper into a small bowl.
Brush each lamb cutlet with the mustard to give a thin coating.
Coat each cutlet in the porcini and thyme powder.
Heat a large frypan over a medium-high heat. Add the oil and butter and allow to heat until foaming.
Fry the cutlets on the thin, fatty edge for 2-3 minutes until crispy. Turn and fry on each side for 2- 3 minutes each and a dark brown crust is formed. Baste in the pan juices and remove to rest for 2-3 minutes.
Dust with any remaining porcini powder and serve immediately.