Put the garlic, 50 gr. of the butter, the zest, the oregano, the cinnamon and the rosemary in a food processor or in a mortar and mix them until the mixture looks like ointment.
Add salt and pepper and massage the meat with this mixture.
Put the lamb in a bowl, seal it well and put it in the refrigerator for one night or at least 3-4 hours.
The next day, cut 2 large pieces of aluminium foil (1 meter each) and 2 pieces of baking paper (1 meter each).
Take the meat out of the refrigerator an hour before cooking to adjust to room temperature.
Preheat the oven to 150° C in fan mode.
Shape a cross with the aluminium foil and the backing paper (put the aluminium foil on the bottom)
Salt and pepper well the potatoes and put them on the baking paper.
Put the meat on top and cut the remaining butter into pieces and scatter it over the meat and potatoes.
Seal it well and put it in a small baking pan in the oven.
Roast for four hours.
Remove from the oven, turn the heat to 200° C and roast for another half an hour until the meat gets a nice colour.
Remove from the oven, cover and leave the meat for 30 minutes before serving.
Serve with salad.
If you want, roast the potatoes to get a nice colour while the meat is outside of the oven for half an hour.
You can also put other vegetables with the potatoes such as peppers as well as cheese (gruyere).