Well, it's a waste with rack of lamb....but when it's been overcooked, there's not a lot to do but cook it more.
I would make it into a medeterranian sort of stew.
I would Sautee some onion ( half of a medium one) and sweet peppers...maybe 1/2 a red pepper, then all a can of whole plum tomatoes, broken up with your fingers, a 1/2 glass of red wine a couple of cloves of crushed garlic and a small eggplant peeled and sliced or a couple of very small zucchini....stir and simmer until the eggplant is done and the meat tender. I would put a lid on it at first and remove the lid if it's done before it has cooked down to a good thick stew.
i have done that with some tough shoulder chops...and a leftover leg or lamb..
Oh yes and I like a tiny pinch of cinnamon in that!
Linda C

I don't go out for dinner on holidays because, having been in the restaurant business most of my life, I know that the customer load can be 3 or 4 times heavier than regular nights, and far too often, the kitchen (and the servers) just can't handle it. The rack was probably ruined because it was hastily defrosted when they unexpectedly sold more than they had anticipated.

I'm with Linda, just make that lamb into some other dish, that takes a good amount of simmering time to tenderize it.

If the restaurant is one that is usually good, it wouldn't hurt to drop in and speak to the manager about how disappointed you were. Likely, he or she will be aware that they had run out of racks that night, and had to force defrost them (or whatever happened), and may offer you some compensation in the form of a discount your next dinner.

Also, a good manager will appreciate you taking the time, because contrary to what many believe, a customer complaint is a good thing, if legitimate, because something can be done to make it up. What you don't want is for people to go away unhappy and tell all their friends how unhappy they were!