Mix the butter, garlic and herbs, then add the lemon juice & zest. Season with salt & pepper to your liking.
Make several insertions in the lamb, then massage the mixture all over the meat, pushing it into the little cuts with your fingers. Let it rest (you can marinate the lamb for a couple of hours or overnight in the fridge, if you like).
Take the lamb out of the fridge, if it was resting there at least 30 minutes before cooking.
Set the oven temperature to 180°C.
Cut the onion and lemon into 1cm thick rings; the carrot and celery into large chunks.
Cover the base of the large baking tray with the chopped veggies, then put the lamb on top. Pour over the red wine then put the tray in the oven.
Roast for about 60-90 minutes, depending on how well cooked you like your lamb - you should calculate 40 min/kg +20 minutes for a medium finish.
Take the tray out of the oven, place the lamp onto a large plate. Cover it with foil and several kitchen towels to rest and keep warm.
Meanwhile, remove the veggies from the tray, then make some gravy from the cooking juices, some more wine and cornflour.
Carve the lamb and serve with side dishes of your choice, drizzled with the gravy. (You can serve the roasted carrots and onion too, if you like)