My little ones love lamb. As far as how to cook it, I like to make a stew with the meat, carrots, potatoes, some salt and pepper, chopped onion, and a little water. Set it on low for about 8 hours or on high for about 6. It will be falling apart tender, and easy for a baby to pick up and finger feed.
Lamb is higher in zinc and iron (and, in my IMO, more digestible) than beef. I can't stand liver, so I never use it. I think that there are other options for highly nutritious foods. Like PP, if the baby isn't able to finger feed, perhaps he isn't ready to start solids. If he is, but has no teeth, I would probably just give him the bone to gnaw on, and focus on encouraging him to BF exclusively for a few more weeks/months.