lamb leg steaks are like chops, designed to be cooked quite quickly and eaten a little pink so they are tender.
If you are going to slow cook them, they will initially go a bit tough and then go tender again once the reach falling apart stage.
Lamb shoulder is a better joint to slow cook, but will ooze off a lot of fat
I would trim the visible fat preferably after cooking, so you get the flavour from it thru the cooking process, but don't eat it. cook it like a chap, fried for 3-5 minutes per side depending on thickness and desired pinkness