Bring the leg of lamb to room temperature. If it was in the refrigerator, take it out 30 minutes to 1 hour before cooking.
Mix the softened butter with the sweet paprika, and the rosemary and thyme leaves.
Prepare the lamb by trimming any excess fat with a sharp knife and patting it dry with paper towels. Season with salt and pepper.
If the leg is boneless, insert garlic cloves inside. If it’s bone-in, make two small incisions near the shank and insert two garlic cloves.
Rub the entire leg with the butter mixture, massaging it well so the flavors penetrate the meat.
Tip: You can do this the day before or a few hours ahead and refrigerate the lamb, wrapped.
Preheat the oven to 200°C (392°F).
Place the leg in a roasting pan, meaty side up, and place the bones (if any) alongside.
Roast in the preheated oven for 15 minutes at 200°C (392°F).
Then reduce the oven temperature to 180°C (356°F) and continue roasting for 45 minutes for medium-rare or 1 hour 25 minutes for medium-well.
Every 20 minutes, baste the lamb with its juices and turn it so it cooks evenly. Add a splash of water each time if the juices are drying up or reducing too much and risk burning.
If the lamb browns too quickly before the end of cooking, cover it loosely with foil and continue roasting.
If you have a meat thermometer, check the internal temperature of the lamb leg. For medium-rare, it should read around 60°C (140°F); for medium, about 65°C (149°F); and for well-done, up to 75°C (167°F).
After cooking, let the lamb rest for 15 to 30 minutes under a sheet of aluminum foil to allow the meat to become more tender.
Collect the cooking juices from the roasting pan. If needed, add a bit of water and bring to a boil for one minute.
Drizzle the lamb with the juices and garnish with a few sprigs of rosemary.
Serve with mashed potatoes, flageolet beans, or green beans, for example.