The leg of lamb should be at room temperature.
Preheat the oven to 150°C / 302 °F (fan-assisted). You can also place it in a cold oven.
Mix the softened butter with the sweet paprika, the leaves from one sprig of rosemary and one sprig of thyme.
Trim the leg of lamb, remove any stamps (ink markings) that may be on the meat, and cut away any excess fat around it.
Lightly salt the inside and season with pepper.
Insert the 4 garlic cloves if your leg of lamb is boneless. If it’s not boneless, make a small incision in the shank (the meatiest part) and two more in the leaner part of the leg, then insert the garlic cloves.
Rub the entire leg with the softened butter, massaging it well.
Reform the leg by securing it with wooden skewers, as shown in the photo below.
You can tie it with kitchen twine or sew it, but this method is easier.
Place the leg of lamb, meaty side up, in an ovenproof dish, and lay the bones next to it.
Place the leg of lamb in a preheated oven at 150°C (300°F) using a fan (convection) setting. If you don’t have a convection oven, don’t worry, conventional heat works just fine.
Every 20 minutes, baste the leg with its juices. You’ll need about 2 hours and 30 minutes of cooking time for tender, juicy meat.
Turn the leg 2 to 4 times during cooking, basting it each time. This will help it brown evenly.
If it browns too quickly before the end of the cooking time, cover it with a sheet of aluminum foil and continue roasting.
If the juices begin to reduce too much or start to burn, add a small glass of water.
Ten minutes before the end of the cooking time, place the dish with the baby potatoes and mushrooms in the oven.
After 2 hours and 30 minutes, remove the leg of lamb from the oven and cover it with aluminum foil for 15 to 30 minutes. This will make the meat even more tender.
Transfer to a serving platter and surround it with the roasted potatoes and mushrooms.
Collect the pan juices. If they’ve reduced too much, add half a glass of water and boil for a minute in a saucepan.
Drizzle a little of the juice over the lamb, top with a sprig of rosemary, and serve.