I was intending to cook roast beef with a joint of sirloin, but unfortunately my wife has returned from the supermarket with the sirloin already cut into slices (it wasn't her fault, and I don't know why the butcher sliced the meat). My question is: would it still be possible to roast the sirloin as if it were one joint or should I try a different cooking method?
Sirloin is a better steak than a roast, but if that is what you want...
Sear the cut surfaces in a hot pan, and then quickly assemble it into a roast block and twine/skewer it together. Don't go crazy, it is never going to be perfect
Roast as normal, baste a little more often than normal, and maybe trim a few minutes off the time
Now the fun with this is that you can flavour the slices before assembly, garlic, herbs, what ever you like. It's like having a stuffed roast, but already in serving portions. Many people cut pockets into their roast to stuff/flavour it, now you have it done in a large scale
You could try tying it up into the original shape, but the trouble is that you've got cut surfaces which have been exposed to bacteria, and which will not reach a high enough temperature to kill them unless you're going to cook it to death, so I wouldn't recommend this. It seems you've actually got sirloin steaks, so why not just cook them as such?