With lamb and beef joints, I think they either need to be cooked slow and long, or until medium-rare.
Choose a cut with fat marbled throughout. Not enough fat makes it quite dry.
Don't worry about searing, it doesn't make enough difference.
I take it out the any wrapping the day before, generously salt every side, then put back into the fridge in an open container.
On the day, take it out an hour before coming, so it can reach room temperature.
I roast it on a bed of onions, carrots, celery etc.
Then either cook at 200c until it reaches the right temperature for your preference. Use a meat thermometer.
Or cook it very slow and long, and then it'll fall apart.
Then, rest at room temperature for at least 15 minutes before serving.