I also have a beef chuck roast I was thinking of grinding and adding in with the lamb to stretch it a little further and maybe help with the texture/fattiness of the burgers.
i just had lamb burgers for dinner tonight. i usually use 1/4 or 1/3 lamb to ground beef...tonight was chuck. spice it up with ground onion, lots of chopped parsley, s&P, worsteshire sauce, minced garlic, some oil and form them big and thick w/depression in middle. if you have too much ground lamb, portion it out and wrap well and freeze and take out as needed for more burgers.
i forgot to add the dried rosemary, oregano and cumin seeds.
Lamb burgers are the greatest!If you're going to grind it fresh at home, you'll obviously trust the person who prepared the meat (you), so you can serve it medium rear, which eliminates the fat issue. If you want it to " look like a beef burger" for guests sake, incorporate a little ground pork. This will counter the lack of fat content. But, I'd go for the medium rare burger.