Mix together all the chopped herbs on a plate. Spread mustard on both sides of each noisette, then press into the herb mixture to coat evenly.
Make the tangy butter beans. Heat the oil in a frying pan, add the onion and fry gently for 5 minutes. Add the remaining ingredients to the pan and cook gently for 5 minutes.
Meanwhile, secure the thin end of the lamb around the base with a cocktail stick. Place on a foil-lined grill pan and cook under a preheated hot grill for 4 minutes on each side or until cooked but still slightly pink in the centre. Serve immediately, surrounded by the tangy butter beans and accompanied by mixed salad leaves, if liked.